I am, i use it too since there is nothing better, but it has disadvantages:
Creep.say has big say bubble that hides name, other creeps and others creep bubbles
creep.say as of now is const cpu (0.2) on each call, but it's NONE cost on PTR
And It makes hard to point at something like where creep is going, you can shout coords but coords are inconvenient to find in room and id's are even more inconvenient and not fully fit in bubble. (Colors\Prefixes don't help with that either, but drawLine does)