Looking at the skill... is there any point in having more than 20 ranks in it?

Less, actually, since the minimum roll is 1 and your intelligence bonus applies to the check. Just put on some intelligence boosting items prior to crafting, and you can have it effectively mastered for more like 15 ranks.

I'm not a big fan of setting traps in the NWN games, and to be honest I wouldn't bother with putting any ranks into craft or set trap. Still, if you want to give it a shot it is functional.