If I can influence the decision in a group, I will always go for a second striker. Firstly, they're the most popular role, so you always have someone wanting to play one anyway, and secondly, strikers make combat go quicker.
I can tolerate a second defender if they're built to be strikerish. A second leader or controller, though, is intolerable as you really notice the difference in length of combats. The extra healing of a second leader is usually always redundant and the bonuses from a controller rarely do anything to speed combat.