The faster the server and the faster the pipeline to the materials the smaller the bottleneck. It is highly unlikely that hosting the footage on the network server would be faster than working from internal drives on your local machines unless your network connection is very fast (fiber) and your server is up to the task of spitting out data at faster than drives do, and the raid is faster than the local drives.
The only improvement you may see is in available storage space. On the other hand, if you are working with USB drives for media on your local machines you may see a great improvement.
So would you recommend to avoid using a server machine and instead perhaps connecting all the Macs to the same external drive(s) via ethernet or firewire ?
You can't connect more than one mac to an external drive. Just the Ethernet isn't as fast as an internal drive. An external drive is only as fast as an internal if you connect through an ESATA connection. A raid is faster than a single drive if it's formatted correctly. A raid over Ethernet is limited by the Ethernet connection. Only fiber through a SAN is faster than a raid internally. I'd put fiber cards on your Macs (assuming Mac Pro's), then put a fiber network together using your fancy server to pump the data. If you can't afford to do that, then hook an external raid through esata to your main rendering Mac or just drop a couple of drives inside and stripe them to raid and put your watch folders there. If you want to use the other machine for rendering copy the files to it.