Right before the big asuki forum hack,
this thread span over 20 pages long. Among fansubbers, there are wealth of knowledge on how to make certain effects using SSA/ASS. And with the growing amount of knowledge sharing and program/tool sharing that's occuring on this forum recently (especially for the past a year or so), that knowledgebase in general is expanding exponentially. Of course, vobsub/vsfilter enables us to put those SSA/ASS script on the video.
The knowledge sharing etc has been going on in even greater amount and in greater length on doom9's forum. (And to an extent, on the individual forums of the frequent doom9 users.)
We can't say the same thing about programs like Premier. That's all - we're just not used to it. And when we have tools that works great for us (it may feel klunky at first, but after a while, some people can do some wizardly works using these freewares), there is little or no need for us to turn to commercial programs.