Excessive Plus is fully compatible to be played without any client-side modifications. That also means that your server will be more popular and filled with new gamers because they don't need to download anything to play on a Excessive Plus server.
Use the /players command to see who is playing with/without the client-side modification: players without the mod are marked with a red asterisk, and even further, players with older Excessive Plus versions are marked with a yellow asterisk.
But nevertheless it's very recommended to play it with the client-side modifications, specially on servers that use non-baseq3 physics or modified weapons times, so the settings can be predicted properly and the gaming experience is better. Additionally, the client modifications include several useful features of its own.