Yeah, I guess there's numerous ways of increasing speech skills. Just talking to people in general, and even buying and selling stuff to merchants since mercantile and speechcraft merged together. With that, speech would increase but not as a conscious effort.
It might be just easier to use illusion to increase an NPC's disposition though. There's a lot more to illusion compared to speech by itself... Intimidate can be replaced by demoralize, dispositions can be increased by good charm spell all while illusion provides invisibility, frenzy and other fun stuff. At least it's better in Skyrim since they merged mercantile with it. I could be very wrong about my doubts on speech. Speech does bring huge advantages to quests since most quests involve interacting with NPCs. Not to mention some funny stuff you might convince the NPCs to do while you're at it. I do always go for speech first in Fallout. Now that's merged with barter, it's even better