I think color neutrality is extremely valuable to becoming great at 3x3. Yes it is very possible to get sub 10, and maybe even sub-8 with Single color cross (or maybe single color + opposite), but there are just so many extremely good cases presented to you when you're color neutral.
Pros of Color Neutrality:
-6 chances at a good cross case rather than 1
-6 chances to find fast/easy x-cross rather than 1
-OLL/PLL stage are not heavily impacted by color neutrality due to the pattern-based recognition system most use
Cons of Color Neutrality:
-Hard to learn if you're already consistently fast with single/opposite color
-Searching for F2L corners on the fly becomes tricky to wrap your head around at first
-Must choose cross color first during inspection, so you have less inspection time for x-cross (in theory)
I think in general, color neutrality is 'strictly better' than not color neutral, as all the cons of CN seem to be based on practice/changing old habits. I can't think of any downsides (assuming equally good with both) to having 6 different chances at a fantastic cross/xcross case. The point made about inspection is relevant because you are technically adding one more step to your inspection phase (choosing cross color then doing the cross/xcross planning), but overall I think that would be minimized with practice. If you look for good/bad edges you can pretty quickly decide which is the best and go from there.
I should add that this is in regards to CFOP and CFOP-related methods. I have limited knowledge on the others and don't feel qualified to speak to color neutrality in regards to those.
Just my 2 cents.
Chris