If you have a unix-y system with a static IP, please join the pool.
If you are on a low speed DSL and is concerned about the extra traffic, you can set your "netspeed" low in the management interface on the pool website and you'll be taken out of the global pool. The traffic is very low, but being in the global pool still gives a few "spikes" once in a while (I'm working on that).
Conversely: If you have enough bandwidth to not mind the spikes, then we really need you to join the pool. Since adding the option the total number of servers has gone up nicely, but the global pool has stagnated a bit. Can't have that. Every day thousands1 of computers start using the pool; we need more to serve it too.
1 I've no idea if that's right; but thousands of new Linux installations a day doesn't sound unlikely, does it?