DarrkPhoenix: Yeah, in NWN2 if you know you're going to be using a keen weapon then the Improved Critical feat is just a waste. Some players may choose to take it if they don't want to be bothered having to buff with Keen Edge often (or don't want to waste a 9th level spell slot on persistent spell Keen Edge), and depending on the module players may want to use limited weapon enchantment slots for something other than Keen, and so will use Improved Critical in place of enchanting their weapon to be keen.

Ultimately it just comes down to there being multiple ways to achieve the same effect, each with a tradeoff, so it's up to you to decide what your preference is.