When I was an SA, the firm really encouraged clerkships. I had considered doing one, but wasn't sure about it. When I got my offer, they just told me to let them know as soon as I did whether I would be clerking or joining the firm the fall after graduation. There was no pressure; the only reason they wanted to know was so they could interview some 3Ls if more people ended up clerking than expected and the practice area they were going into really needed help sooner than 2 years down the road.
Really, the more firm bio's that say, "clerked for judge so-and-so" the more "connected" the firm looks, IMO, and makes it more attractive to clients.