Mark, I think the speed bit depends on the user. I for one can not read music and create abc using a graphical programs where I can copy what I see. I use Noteworthy/ abc2nwc and then some tweaking of the abc to get it right with abcm2ps so it takes me longer than producing a MIDI. On the other hand, I think a sight reader who has got to grips with abc will find they can type abc in very quickly.
Oveall though, I have thought for some time that MIDI is a throughly bad format for songs... You have mentioned many of the reasons to use abc. Others are in addition to MIDI conversions the project may require, all the songs in a 200 song project could also be supplied in one simple text file and that one can also produce high quality graphics from abc.
Also, I don't know the feasibility with the ammount of trafic and demands on processing Mudcat gets but even on the fly conversions may be in the realms of possibility. Here is an example of a project I tried to get going but seemed to die. In this case, I just take a simple abc text file for each child group and extract and process what is requested on demand. It would be a simple step to add the ability to put the songs in any key requested too. The programming required to do such things is very easy.
Just putting ideas around and expanding on Mark's thoughts...