I could be mistaken (and someone please correct me if I am) but I'm pretty sure that they don't mean not to take the check valve off the booster at all, they mean do not use the booster without a check valve.

Reason being is that the check valve not only helps to hold a solid vacuum inside the booster, it also protects the diaphragm inside the booster from damage should your engine backfire through the intake.

A backfire could pressurize the vacuum tube going to the booster and blowout the diaphragm. The company that sells the boosters just doesn't want to replace a bunch of them for free just because someone decided to bypass their safety feature.

Hope this helps.

Jeff

Some sound advice should you find yourself at the controls of an aircraft...

"Try to stay in the middle of the air. Do not go near the edges of it. The edges of the air can be recognized by the appearance of ground, buildings, sea, trees and interstellar space. It is much more difficult to fly there."