In a site or office a big as yours I would recommend separating clients (workstations, etc) from your servers. Depending on the trust levels, you can join those networks with a router (a layer 3 switch) or a firewall. The main reason for having this separation is it gives a point of control and inspection if you need to. You can block individual IP addresses, or protocols by TCP port, as well as report on that activity. Even if you don't enforce anything normally, you have the capability to it easily when you need to. If something abnormal is happening in your network, and you want to start chasing things down, you can do that most easily at a router or firewall interface. Of course it also allows you to have servers that have restricted access under more severe control, for instance your finance servers or backup servers.
These days even mid-range switches have layer 3 routing capability, and all are able to perform routing as fast as regular switching with their custom ASIC chips.