Well, I'm an intensive client-side programming adept: I propose JS everywhere. But I'm unsure this helps a lot because:
[list=1]
Some people have JS disabled. Means we need bbcode parsing available both ways, which makes coding and maintenance more complicated.
Search engines don't understand bbcodes. Means the old way is required for them too.
Some bbcodes, like hidden text, cannot be done client-side.
It slows down page rendering by web browser, which may cause negative visual effects on slower client machines.
So, it's an interesting idea but I'm unsure it benefits over complexity it brings. More pros?
May the Source be with you!