We've had drummers who provided the free rehearsal room, singers with a PA (to run all the instruments through as well as the vocals),band members with vans, guitarists with lots of friends (for a ready made audience) or website-building skills.
My main contribution, and those of a number of gig-getter readers, has only ever been in being able to market the band and bring in plentiful supplies of gigs when the other band members wouldn't know where to start.
As well as checking out the goals, ambitions and the amount of time to play potential band members may have, you might want to try to explore what they may be able to offer "off stage" as well before you decide on their suitability. Just make sure they can perform as well of course...
http://www.gig-getter.co.uk