i have a separate tank for my inverts as my other fish tank has large fish that will eat them. and what exactly is "good" about vac? is that just what you heard and what you have done so you stuck to it?
The good thing about vacs, is that (of course) they will remove all the cruddy stuffs that slip between all the gravel (uneaten foods, decaying plant matter, fish waste, etc).
While this gunk does break down and the plants will enjoy the nutrients that it releases, over time, the build up can be bad for the fishies as it will deminish the water qualitiy quicker.
Now, I'm definately not saying that you must or should be vaccing everytime you do a wc. For that I would say BAH! What I am saying, is that every once in awhile...sure...not such a bad idear.
And no, this is not what I've heard or just a routine that I myself have gotten in to. We all have "heard" what you "should" do, what you "need" to do, etc, etc, etc, blah, blah, blah. What it boils down to is what you learn along the way and what works for you and your tanks. Your environment is unique to you.
IME...here's two particular examples from me personnally. Different substrates (river gravel and SMS). Heavy plantings and fair stockings (all small community fish so it probably doesn't compare to your bigger tank) No gravel vacs. And of course, the smaller the tank, the greater impact.
1) Gravel (this was mentioned above with the anaerobic bacteria). When doing some plant maintenance in a 29g, I stirred up a bunch of gunk. Decided to vac. When I plunged the vac into the gravel, you could see tons of bubbles being released from below. And these weren't small air bubbles. Some were rather large. That was a bit of shocker to me. Figured the gravel would allow more space for it to escape to the surface. Now, once every so often, I will vac around the open spaces. Not all time mind you, but enough to keep things in check. Turns out to be about once a month or a bit longer.
2) My second example relates to the TDS that I mentioned. Again, I don't know what effect (if any) it has on fish or plants, but I know it's bad for shrimp. 10g tank, SMS substrate, heavy plants and shrimp. No vacs at all. Shrimp started out fine. Breeding and expanding their colony. Then over time steadily decreased without cause (or so I thought). Water parms always showed good. Research led me to TDS and I got a tester. Was off the charts. A couple good vacs later, it was back to acceptable levels. I now make a point of vaccing the open areas of this tank once a month as well.