I think the benefit of working at startups is similar to the benefit of working for smaller companies in general. With a small team you'll get to touch more of the stack, and maybe even parts outside of your job domain altogether. In my first job I was the 2nd developer, the first was the CIO. In addition to writing most of the code for this project, I set up the entire production network including load balancers, database replication, firewalls, etc. I even picked out our hardware and physically installed it in the rack we rented at a local ATT data center. Now that's an extreme case obviously, but you will never get anything close to that broad of a base of experience working for a BigCo.