VCAs. The thing that new builders neglect...until they discover just how much their build SUUUUUUUUUUUUUUUCKS without them!
VCAs have so many uses, it's nuts. But then, you have to remember that the initial idea for modular synthesis partly came from analog COMPUTERS, which are very dependent on voltage relationships with/between op-amps. Since all the analog computer voodoo lies in how you use those, it makes perfect sense that we have a similar circuit in modular synths.
Anything that needs level control for which you don't happen to have several extra arms and hands, that's where VCAs go. Audio and CV/mod levels are the basics, though. You can also use them as amplitude modulators, by feeding an audio signal to both the signal AND CV inputs. Lots of clangers and yowls! A couple of them make up the guts of stereo autopanners. And performance mixers are usually jam-packed with VCAs to control levels, panning, AUX send/returns, and so forth. And if you want to scan/xfade through a VCO's waveforms, a quad VCA + a quadrature LFO are a must. And that list goes on...
I think some of the problem with them lies not just with the "boring factor", but the term "amplifier". People look and think "well...I've GOT an amplifier and it's hooked up to my speakers already, so I don't need all of these amplifiers...right?" Wrong. Whatever the cause of this, it's a HUGE trap and part of the "boring module" issues that people run across...which I've termed "sexy module syndrome". These modules don't come with a plethora of knobs and lights and so forth, so people load up on the "better" modules that DO have all of that...then wonder why their $7k+ box sounds like total ass, give up on modular, and yet another cab gets consigned to the closet. It's not necessary...people just need to do their homework about these instruments BEFORE grabbing the Magic Plastic and burning yet another expensive hole in it.