I say no ranking system unless......
Unless it's something that EA does with their regular ranking system. In other words, if EA at some point says that they are open to the idea of adding the mods to the ranking stats, then fine. Otherwise, if it's a standalone ranking system that is apart from the basic 2142 rankings, then no. The only reason I would say yes, if it were part of the official rankings is, then it would draw a ton of players.
In my opinion, the rankings have mostly increased players interest right at first, but once we all get to a certain level (got all the unlocks) then "who cares". The down side to rankings are more cheaters/stat padders and fewer people that will play mods ("cause mods are unranked").