I think buzzing without the mp is helpful. My instructor not only recommends it but showed me how to do it. It takes a relaxed mouth and with some practice, you can buzz just as high as you play on the trumpet. You can even buzz pedal tones or make a siren sound (much easier with the MP).
When my cheeks were puffing out (not sure where that came from), we fixed it with buzzing w/out the mouthpiece. And I was also able to fix a weird stuffy/airy sound I went through by going back to the BASIC BUZZ (w/out the mp). Since I learned how to do it, I can get a very clean buzz w/out the mp ..... but then something was different with the trumpet. When we figured out what it was, the problem was solved.
I use it as part of my normal warmup ..... loose lip flapping, then buzzing w/out the mp to make sure it's a nice, tight sound, then with the mp to refine the sound. And on to the trumpet.