I'm not sure about that. They certainly shouldn't locked into NL only because that could severely limit their ability to contribute to the team in BH and CC or C&D defense matches, but I agree that they should have a heavy NL focus in their design. I'm very disappointed at the lack of viable NL options for Infiltrators outside of the Wu and fulton cannon (Scouts get the tranq sniper, Enforcers get the Sleep gas launcher, and we get a shotgun-shaped paper weight).
Plus reforming one of only three classes to be permanently unable to perform open combat would require rebalancing the others as well.
If they were going to put that kind of effort into defining the roles I feel like Infiltrators should have SMGs + Pistols as lethal and NL options (add the stun SMG), you can give the shotguns* + LMGs to Enforcers (Lethal only) and ARs + Snipers to Scouts (Lethal and NL).
If you wanted to take it to the extreme, they could remove Enforcers ability to fulton as the 'combat' class (Bomb suit and heavy weaponry don't scream "I'd rather not hurt you, here's a balloon" to me), and give Infiltrators the ability to carry two support items (NL-Only, otherwise just one, and don't forget about your carry weight!) for increased tactical play without dedicating two skills to it***. I'm not really sure what change to give to Scouts here, they're probably fine as-is since they're a sort of middle ground class.
But I digress. To me the problem is that NL isn't rewarding enough to justify the difficulty. It's too easily countered by two enemies simply standing in a formation where they can see each other, or a lone sniper keeping an eye out for balloons. Infiltrators that come in with ONLY NL equipment tend to be limited to picking off loners/campers or laying traps unless they have a good buddy supporting them the whole way. If an infiltrator sneaks up on your squad unseen it should be terrifying, not laughable because they're so squishy you barely need to aim to drop them.
...I don't even wanna talk about how broken CQC is with the game's lagg right now.
*Perhaps they could have SMGs in the primary slot since the knife will take secondary 2 when it's added
**I'm not against Infiltrators getting the stun shotgun (and tranq sniper =p) to add to their NL capability, but shotguns are typically heavy weapons, not silent ones.
***And in the vein of increasing Infiltrator's tactical style I feel that things like the Fulton 3's "fulton punch" should be replaced with something like "rescued fulton victims are KO'd again upon hitting the ground" and the whole stealth camo tree should be reworked to be more of an infiltration tool than a camping enhancement. Also what good is interrogation if I need to dedicate my skill slots to Interrogation 3 to share the information with my team? As the sneaky-sneaky player that information isn't nearly as good to me as it would be for my team's Enforcers.