If you let it go, it can get bad, I suppose. But cleaning out a manifold every 150K miles isn't so awful. Shouldn't affect performance much, either way. I won't even address the technical arguments, it should be sufficient to know that removing it is illegal, and can get you a hefty fine.

To test, apply a vacuum directly to the valve with a Mityvac. It should hold without having to continuously pump. When you release, you should hear the valve click closed. There's not a whole lot inside, if the diaphragm holds vacuum, just take the valve off and clean out deposits.

A better test is to add a y fitting at the valve, and run a vacuum gauge into the car. Watch it as you drive. Vacuum should be zero at idle. Just off idle, it will peak at around 15 inhg. Then it will move smoothly in proportion to speed and load. The vacuum will return to zero in high speed/high load conditions. If it doesn't, then replace all the rubber elbows involved in EGR (it's actually a good idea to replace all the rubber elbows throughout the engine compartment every so often).

If the valve always shuts at a particular load condition, a likely cause is a problem with the air mass sensor, which is located in the air cleaner. It's a flap that opens against spring pressure: the wider it opens, the more air is going in. The sensor part is a coil of resistance wire, which may have a dirty or broken coil.