It depends on the situation.
If you've got more than one site, then certainly. Clients should always have fast access to a GC.
If you've got one site, but use Exchange 2000 or 2003, then yes as well (you'll want the redundancy, if the GC goes down, Outlook clients start borking).
It also depends on the scale of your forest and the number of objects. The larger the forest, the larger the replication task. You'll need to start customizing replication schedules across sites, and replication traffic becomes a factor.
If you've got a small, single forest domain with a small number of objects, 2 GC's is going to give you more benifits (redundancy) than problems (replication traffic will be minimal).
Hope that helps.