FF_ANIMATE support was coded in because of Super Sonic. I didn't want to have a special standing state just for the spine animations done for FSonic and FFSonic, since it was important that the nextstate of S_PLAY_STND be S_PLAY_WAIT. To that end, I added the FF_ANIMATE thing so that same-anim frame incrementing could be done without commandeering nextstate.
Also on the topic itself: You can modify mobj.anim_duration directly, you know that right? Modifying state stuff isn't net-safe, but anim_duration is.