Hope you don't mind me nerding out here - I've had an MT-32 since the late 90s and it's always been one of my fave devices, so I've kind of passively absorbed a ton of info about period MIDI stuff.
This is also often how I feel about "enhanced" DOS soundtracks, and for the same reason: anyone composing for the PC would have been an absolute fool to target the Roland, a thing that like 50 people had. People had adlibs and things with adlib compatibility, and I would be astonished if hardly anything wasn't composed for those first, then "upgraded" for Roland and GUS after the fact.
You'd be surprised! Often that was definitely true; for example I've got a copy of Ocean Software's in-house tracker, where the same program was used to compose for Amiga/Adlib/MT-32/SC-55/SNES, and it's very clear that (depending on the game) Amiga, Adlib or SNES was the primary and anything else was a conversion.
But the MT-32 and SC-55 were the lead more often than you'd think. Part of that's for economic reasons - take a look at old Sierra catalogues, and you'll see that they'd actually sell you an MT-32. They were a dealer for it! The first Sierra soundtracks that support the MT-32 came out at pretty much the same time as the Adlib, so it wasn't a question of composing for the device most customers had - it was MT-32 vs PC beeper or PCjr as the device most people had. And if you were going to target one device as the primary, you might as well target the one that you're selling. Makes a better selling point.
At other companies, sometimes people targeted the MT-32 over Adlib just because it's more fun to compose for. Composers are artists just like anyone else - it's more interesting to work with the more expressive hardware even if you know fewer people have it. Your boss might not want you to spend that much time on the fancier hardware, but you'll have more fun working with it. It also probably integrated better with MIDI-based composing environments, making it easy to start with a full MIDI synth and pare back to Adlib later. Not everyone targeted a MIDI synth first but some people sure did.