Currently, I see almost no reason to ever play brute instead of raider of juggernaut. I feel like the juggernaut is clearly better at being a constant presence around the enemy flag, especially with his quick health regen. Also, the raider is clearly better at dueling all of the light defense that try to retake the enemy's base. The ONLY thing that the brute has to his advantage is the fractal grenade, which is really only superior to the raider's EMP grenade in very few situations, usually inside small bases. Also, the utility of camping in the opponents gen room is pretty questionable, especially at a competitive level.

I don't think that the brute has a clearly defined role, much like the soldier. It's not necessarily a bad thing that the soldier isn't clearly better at any one thing, however he's DECENT at basically everything. If you want a class that's good at switching roles quickly to go where's he's needed, the soldier can be effective, and get across the map quickly and do that role well. If you want to call "all around flexible quick response team" a role, that's what the soldier is.

The brute however can't. He can't get across the map as quickly, nor is he even comparably good at doing any of the tasks he would be required to do as a "jack of all trades" heavy offense kind of class.

In my opinion, the only good role that the brute can do effectively and somewhat uniquely is exactly what you said, being a constant presence on offense. This is pretty similar to the raider, except less mobile, more resilient, and worse at dueling. If I were on the design team, I would make a new primary weapon for the brute that is more of a short to midrange MIRV launcher or a heavy grenade launcher. This, displacing the spinfusor, would make the brute hopelessly immobile, however would make him much more effective at area control inside bases and near the enemy flagstand.