well, you must take a look to the last two,I play with them in more serious games :)
but,when I say important, I don't mean that shot block skill must be at the same level that IS or ID ,it must be at a decent level.you say that 2 block's per game is not so important , but, in that order of thinking, we can say that FT also is not important, because a player will score 2-3-4 FT per game :) but when you play with a strong team, and every point is priceless,then this 2-3 block shots or FT can be very useful.And because this is a team game, in one match not just 1 player will make block shots,they will be more,and the same situation is with FT:). Like GM-Katchafire Collie said,right now the block skill still has a limited role, and it is not(yet) a factor in winning a game... but who knows what will be in the near future :)
this is just my opinion and it may be wrong :-P