The problem with unlocking faction weapons is that it merely creates a short term illusion of asymmetry, but does nothing to benefit gameplay. If there are faction weapons in the game, then there has to be a reason for it. If you have a good reason, then the question is: Why ruin the faction exclusivity of the weapons? If the reason is a weak one, then why have exclusivity at all?
With faction weapons, there’s no in between. This isn’t to say you can’t have both, you can. It’s just that you can’t bridge the gap between the two. Battlefield 2 had both systems - each faction had exclusive weapons, but they also had global weapons that could be used by all factions for their respective kits. Two separate systems in place to offer their own respective benefits, but more importantly those benefits are consistent throughout a player’s multiplayer career.
By blending the first system into the second, you diminish consistency and remove player choice. If a player enjoys faction specific weapons, then they only have this until x requirement is completed, and a false sense of it as well. If a player hates it, then they have to deal with it until x requirement is completed and their experience is diminished until this occurs. It’s just not a good solution nor a good compromise.