The advice against altering your pH, is because if not monitored carefully, you could have swings in either direction from water changes, etc, which could create chaos very quickly. It just adds another thing into the mix of fishkeeping. A general rule of thumb for easy fishkeeping is to keep fish that are attuned to your water conditions, as they are, so you don't have to make any changes to your pH and your fish don't have to adapt.

I've had trouble keeping some fish in my tanks that, at the time, were a little more alkaline than they are now (because of my decor). My Serpaes and Black Phantom Tetras kept dying one right after the other with different symptoms. I chalked it up to bad breeding/mutations or a disease I couldn't pinpoint (Neon Tetra Disease for one). It wasn't until about a year later where I thought pH/GH might have something to do with it. Many tetras (including these) prefer a more acidic environment, so I'm almost certain that this had something to do with their immune systems being compromised.