Carl,
Pros
- Centralised point of control and administration (firewall rules, IDS etc.)
- Possible lower cost (1 x 100mb internet connection is probably cheaper than 10 x 10mb internet connections)
- Possible lower hardware costs (1 x chunky firewall versus 10 x smaller firewalls)
Cons
- Latency - site internet traffic would have to traverse your WAN and then out of the central site adding latency
- Single point of failure - if your main site internet link goes, everyone loses internet access
- Rule complexity - your central site firewall will need to be configured for rules affecting all of your sites.
Personally I like this design model as it gives you a greater degree of control over your network.