I'd say it has more to do with an increase in general balance. My choice was up in the air until I hit the final battle. Celes, Setzer and Shadow were all in the running until Locke turned a very tough battle into a borderline cakewalk.
The balance that has been achieved over the past few versions has a sort of cascading effect. More balanced (usually less difficult overall) bosses and enemies allow a wider variety of builds to be viable. At the same time, other characters and Locke's other builds have been (mostly) brought up in power. This leads to people playing with non-Phoenix Locke, which is totally fine, but it leaves them less familiar with the power of Locke's X-Magic.
I feel like I made my point, but I want to make sure I point out that everything I'm saying is positive. Text can be woefully inadequate at communicating tone, and as I read what I wrote, it sounds much more negative than I intended. Can't wait to see what new character builds become viable in 2.0! (Dog Farts, yo!)