I prefer best mix (or whatever the guy is selling cheapest on the streetcorner) when I use nitrox, which is pretty rare these days. Air, it turns out, is often adequate for my needs.
For me best mix includes consideration of:
The need for extended NDL's or limiting nitrogen ongassing (same thing but looked at two different ways).
Nitrogen offgassing
The expected Max depth
The potential for exceeding Max depth
For a hard bottom dive to 100' where I need more NDL or I'm doing repetitive dives 32% might be best mix. On a wreck dive where the deck I plan to dive on is 100' but the bottom is 130' perhaps 28% might be best. If I'm altitude diving and want to limit my ongassing and don't need depth 40% might be best. I may stock air in my stage/pony, 32% or even 40% depending on how I see it's function.
I understand the rationale for standard mixes and don't argue against anyone using that method but I think it can lead to mental laziness in regards to dive planning if one is not careful. I also think it's a hard sell to nix computers in favor of BT's and ratio deco then insist best mix is too edgy for rec diving.
Your gas mixture is just another tool in your toolbox. Some people like to use a lot of different combinations of tools and some people just like to stock a hammer screwdriver and crescent wrench. Whatever you choose, just make sure you know how and when to use it.