What are the reasons one wouldn't run Small Business Server?
You can't create a trust relation with an SBS server domain and another, existing domain.
We migrated from SBS2003 to individual Server 2003 and Exchange 2003 servers for this reason. We found that the migration path is weakly supported by MS, and I was worried to death that it wouldn't work. We invested in a pack from www.sbsmigration.com to help us through it, but ended up not needing it.
There are basically two reasons not to run SBS.
- You need more than 75 licenses (total of user + device CALs).
- You need to run a line-of-business application that explicitly doesn't support SBS (these are quite rare, so its unlikely to be a problem).
Aside from that, I cannot think of any compelling reason not to run SBS. There is a restriction of a single domain in a single forest, but there should really not be any need for more than that in a small business.
SBS2008 (and SBS2003) is limited to 75 users (or desktops, depending on whether you opt for per user or per device licesning) in total. Beyond that you are looking at Winidows Essentials Server.