Cons
Well I am not really against this idea, I will give some feedback from my point of view after running a competitive Tribes team for several years. Running a team in Tribes 1 demo, where we played 8v8, there was pretty much zero stress on roster, and it wasn't even really an issue. If competition is slated to be at around this size, perhaps 10v10, a cap would work without many cons I think.
However, if you go like Tribes 2, and have 12, 14, 16 man competition, adding a cap will just be adding another roadblock in the way of building a competing team in my opinion, and while it may add some competitiveness at the very top, it will make it even tougher to field a team on the way up. That is because for any team it is hard to have all good, dedicated, consistent players on your roster, and so a lot of depth (read: lots of players) in the roster is needed; I have had as many as 30 players on our T2 roster at times, and that still has left us struggling in practices, scrims etc for numbers. With a 30 man roster you can of course be expected to have enough for matches. But what about practices, meetings, scrims, all the other things needed for a team's health? That is what a cap might hurt if the roster is severely limited. And if a team has trouble with practices, scrims, meetings etc eventually, that will impact them in matches too, and might prevent them from even competing.