If you have two instruments and both give 'm separate reverbs, it's just going to sound unnatural. "Sound quality" is a weird way to describe it; it's more of a "this is something you can't ever hear naturally". Mixing is about the suspension of disbelief, and unless you're live-recording an orchestra with a pair of microphones separated at a distance equal to your ears in a seat somewhere in the venue (which is -still- an approximation because your ears work different from microphones), you're always trying to "stylize" things to paint a scene, send a message, or perform illusionism. It's like a render of a scene; your eyes don't focus on two distant things at the same time and adapt continuously to the light around us (and fill out a hilarious amount of details outside of your focal point that aren't really witnessed - just painted in there by your brain).
With inserts, you're actively decreasing the volume of the dry signal while increasing the volume of the dry signal. It's a crossfader.
With sends, you keep the dry signal at equal volume while adding more wet signal - and as Jorito says, you should have the wet to 100%. It's two separate faders.
It's about the ratio of the mix plus multipliers. With inserts set to 20% wet, it means the dry signal is multiplied in volume by 100% - 20% = 80% (0.8) while the wet signal is at 20%. With a single wet/dry knob, you can't have a scenario where dry is 90% and wet is 40% or something - the sum always has to be 100%.
With sends, the dry signal is left at 100%, the wet signal at 20% - the sum of this (120%) is the "new" 100%, and if you'd scale everything back proportionally (100/120 = 0.83), it's comparable to the insert's dry/wet as 83%/17% in terms of ratio, but both are louder.