When anyone says something like 'I have yet to hear a valid, coherent argument' against something, what they mean to say is 'no matter what you say, I will claim it to be invalid and incoherent as my mind is already made up'.
Maybe that isn't the case here, but I struggle to believe that someone can come to these forums and make that claim.
Specialists could (heavy emphasis on could) be like a sub class within a class, however how anyone could claim the current specialist situation is like classes in any way is beyond me. What makes a class? Previously it was gun, gadgets and armour. Specialists can have any gun, any gadget and their armour tells you nothing about them.....
Now, most of the issues with specialists can be amended to make them less of an issue; things like having the same specialists on each team messing up enemy recognition, not knowing who is carrying ammo/medic kits, one specialist being better than the rest. All specialists looking the same as each other. With UI tweaks, faction specific colours, customisation and ongoing tweaks to specialists abilities this will evolve and could be fine, in time.
However, some things may be harder to solve, like how do you encourage the use of ammo/medikits over more fun/solo oriented gadgets I e. RPGs? Classes restricted your choice and even non-team oriented players would likely have to run a beneficial gadget that they may even use once or twice. The beta, not full game I know, had an overabundance of Mackay's and RPGs... Hopefully the full game doesn't settle on to such a meta. Multi-class guns, rather than specialists, would keep to those restrictions forcing players to pick useful teamplay gadgets, or even just restricting the gadgets each type of specialist can take - the latter seemingly a great half way house that could make both sides a bit more content.
I also liked classes as you knew what the enemy player could have, from gun type to gadgets, so knew whether to stay at range or rush in or if your tank was likely to be C4'd or RPG'd. This is gone with Specialists. This removes a tactical element that I liked that they can't get back.
The abilities are the biggest issue for me though, as it stands. Some seem very lacking and then you have wall hacks.... I can already see who most of the community will be playing as and with no restrictions on gun or gadget to help balance them, it is going to be a challenge for Dice. This will be a buff/nerf fest for the foreseeable future and including more specialists down the line will only add to this. Who wouldn't want to be the best specialist, with the best weapon and best gadget?