I'm running the server on my own computer, and the vanilla GUI has been extremely helpful for me in performance monitoring. I can see at a glance if a player is experiencing lag because of TPS issues or because my upstream connection is bogged down (happens when somebody changes dimensions or respawns, for example). TPS monitoring itself is much easier as well, since there's an integrated meter that updates itself and tells me a whole lot about my server just by watching it for half a minute. Furthermore I can see if someone (and who) is online while I am not.
Now without a GUI I have to type /forge tps, and then repeat that command 20-40 times to get a real idea of how the current tickrate behaves (if it is stable, rising, falling or jumping all over the place). This ruins my chat backlog and spams the server logs with several hundred useless lines. For network monitoring I have to use the hugely imprecise Windows task manager. And to check if someone is online I either have to boot up my client and log in, or scroll through a console window looking for specific log messages (if it didn't spam them out of the limited console buffer because I was checking TPS). Overall it's just an all-around nuisance to be without the GUI. If there was a -nogui flag to turn it off, why bother removing it anyway? That's just beyond logic.
The only thing that makes it bearable is the fact that MCPC+ removed the performance issues I had (which was the reason I tried it in the first place). So while I lost my monitoring tools, at least I don't need them quite as much as I used to anymore.