Believe it or not, you can still have focus without limiting the highest level one can achieve in support classes. The level you achieve, or number of classes you have shouldn't be the means of doing so, anyways. And if someone can work around this by simply purchasing more characters, how has it worked to help "focus" the game?

As for being able to do everything at 50, that's a rather short sighted statement. The level cap will increase later on, and there certainly will be plenty added for those higher levels. There are, and I am sure, will be new recipes that require multiple Classes, and unless they work to prevent it ahead of time, you are going to end up hitting items in your mastered Class which you won't be able to synth because of a required sub level. It happened in FFXI, so I could easily see it happening again here if they don't pay attention.

Honestly, there isn't a good reason to do it.