What would happen if I removed the basic NetBeui protocol on an NT network and enabled TCP/IP as the lone and default one? Is that advisable?
As with many things, the answer is "it depends." If you have a small network with no routers, NetBeui is marginally faster than TCP/IP (although there is some additional overhead for running two protocol stacks instead of one). It is also the native protocol of all Microsoft OS's back to DOS days. However, in these days of Internet everywhere, you probably need TCP/IP and probably don't need NetBeui. Note also that NetBeui becomes evil in large networks, with its propensity for initiating many, many network broadcasts.
So, if you have a small network, yes, go ahead and take it offconvert one machine, then test it for a while. A very rare number of programs may have problems without NetBeui.