I am curius to know, why do most firm not inerview 3Ls?
Well this year, obviously for economic reasons... as I'm sure you've read, firms already over-hired for 2009, since many offered most of their summer associates... who were hired back in Fall 2007, before the financial crash.
Even in good economic times, re-interviewing as a 3L is an automatic strike against you. If you were a summer associate at a different firm, other firms will assume some combination of the following:
1. You didn't get an offer.
2. You got cold-offered (so even though you're sitting there claiming you have an offer, you actually don't)
3. You're disloyal to a firm that spent upwards of $50k to recruit you.
4. You're a perpetual job-switcher, and therefore won't stay long at the firm.
5. You're not cut out for Biglaw.
Of course, if you work somewhere other than a law firm your 2L summer, you will have a better time with 3L OCI... then you just look indecisive at best, or, at worst, they'll assume you couldn't land a Biglaw gig.... which means they will consider you a "failure."
Unfair, undoubtedly, but this is what 3Ls are up against.