I have been thinking quite a lot recently about the various digital modes from the point of view of which ones are worth supporting in a program. My own software KComm developed for the Elecraft K2 and K3 supports only PSK natively, mainly due to the existence of AE4JY's PSK Core DLL which makes it easy to add PSK31/63/125 support to any Windows program. I can always use Fldigi or another program if I want to use other modes. But you get used to using a particular program and there is a certain challenge in adding support for other modes, which is why I have been spending time wondering whether it would be worthwhile to do so.
The more I think about it the more I tend to the conclusion that PSK31 is the only digital mode that is really worth bothering with. Its usage greatly exceeds everything else including, I suspect, RTTY. It has achieved 'critical mass' which means there are always new stations to work including exotic DX locations. This single point outweighs any of its disadvantages, such as the fact that the phase shift information gets messed up by disturbed paths, it's harder to set up to give a clean signal or that some MFSK modes such as Olivia that have redundancy and error correction are more reliable with weak signals. PSK31 is a narrow band mode and the benefit of that is often overlooked. I think things would be getting pretty ugly in the digital mode band segments right now if all the people who currently use PSK31 were instead trying to use much wider modes like RTTY, MFSK or Olivia.
RTTY. RTTY was the first digital mode I ever used, way back in the days before sound cards, so I have nothing personally against it. I suspect it is the second most popular digital mode after PSK31. I don't think RTTY use has died off, so much as it simply hasn't enjoyed the growth in popularity that PSK31 has. RTTY would be my main candidate for the next digital mode to support in my program. Except that in the last decade or so I have hardly ever felt any inclination to use it.
One reason is that RTTY is a grossly inefficient mode. It occupies a lot of bandwidth for its data rate, due to the fact that it was designed long before computers and DSP were invented and had to be decoded using an analog frequency discriminator. It has a limited character set geared to the Teletype devices it was meant to be used with, and requires a shift character to go between letters and numerals which adds to the likelihood of receiving gibberish if this character is not received whenever it is sent. Consequently RTTY users adopt the sledgehammer approach and run very high power to maximize their chances of good copy. RTTY really is an outmoded mode that has absolutely no advantages or benefits and instead of encouraging its continued use perhaps we should be persuading people to change to more modern alternatives.
MFSK. MFSK was developed by ZL1BPU and IZ8BLY in the early 2000's and I was an enthusiastic user and advocate of it for quite some time. MFSK16 seemed to give more reliable copy than PSK31 at the same power level. It also had a nice feature for exchanging small bitmap images, similar to SSTV. However in recent years it seems to have fallen out of favour and these days you rarely hear it.
Other multi-FSK modes have also come and (in most cases) gone, such as DominoEX, MT63 and Olivia. The benefits of all these different variations on MFSK escapes me, though I'm sure at least some of them do have benefits, however they have never caught on and would probably die out altogether if they did not live on as options in the menus of programs like HRD, Fldigi and MixW providing temptation for every new convert to digimodes to try them.
All these modes seem to achieve is confusion and a proliferation of "What's this mode?" threads on QRZ.com. There just aren't enough users to make many contacts with them, and anyone calling CQ using one of these modes is heading for disappointment unless they happen to be spotted by a PSK31 user who manages to guess what mode it is and takes pity on the poor caller. These modes should really be consigned to the bit bucket of history.
JT65A. A mode currently enjoying a surge of popularity on HF is JT65A, thanks in part to the easy to use JT65-HF software written by W6CQZ. I tried this myself recently and made a couple of contacts before starting to wonder what was the point of it? As Wikipedia explains it, JT65A was developed to allow contacts to be made over slowly varying paths where signals are expected to be weak, such as EME or long distance troposcatter contacts on VHF. Such contacts are likely to be prearranged skeds over paths where no other mode except high power CW is likely to make it.
However the use of this mode to make random contacts on HF seems pointless. People frequently seem to be making contacts when signals are strong enough that the contact could be completed more quickly, with the exchange of a lot more information, using PSK31 or one of the other digimodes. Signals on HF tend to vary quite a bit, so that even if they are very weak some of the time there are periods when they are strong enough to support "regular" modes. Do the problems that JT65A was designed to overcome actually occur on HF? I don't think so.
Conclusion. The conclusion I came to after the ponderings I have tried to summarize above is that PSK31 is really the only digital mode worth supporting if the aim is to make the maximum number of contacts. RTTY would be worth supporting for legacy reasons because it is still quite popular, although nowadays it seems to be little used outside of contests. From a technical perspective RTTY is arguably a mode that ought to be phased out.
The other modes I listed - and some that I haven't - are so esoteric that that they are not worth bothering with because there isn't a critical mass of users to ensure a good number of contacts. These modes may be interesting to try out, but as such they ought to remain within the realm of their own specialized programs - much like the original IZ8BLY Stream MFSK software of a decade ago - so that users are forced to make a conscious decision to use these modes and hopefully find out a bit about the purpose behind them, instead of just picking them from a menu because they are there.
No doubt there will be many who read this far (if anyone reads this far!) who will disagree with my conclusions. But from now on I expect to stick exclusively to the PSK modes on the HF bands and stop worrying about the other twiddly noises I occasionally hear on the airwaves.