So, I have also been playing another server recently, which also had a transmog system - the difference (among other things) was that you could transmog weapons far more extensively than on Frostmourne / Lordaeron.
Specifically, you could transmog a 1H model, onto a 2H, and vice versa. This effectively let you play a pre-Titans Grip Fury (even if only aesthetically).
I personally feel this would be a good chance to add to the already existing system (those who do not like transmog, can turn it off wholesale anyway), so I don't see any compelling counter-argument from a meta / need to know position; it would only give players more flexibility in their transmog choices.
I mean, you had to be able to use the item class anyway; so if you can't use a 2H, you can't use that anyway.
But let's be honest here; the issue with Rogues and 2H is a moot point since Blizzard absolutely ruined Fury and turned it into a clown class with Titans Grip anyway.
It's already ridiculous enough, I don't really see that as much of a counter-argument.
I mean, you had to be able to use the item class anyway; so if you can't use a 2H, you can't use that anyway.
So this would mainly benefit DK's, Warriors, Paladins.
What are the cases they'd swap between 1h and 2h?
- UH/Blood DK's would go for 1 handers? I doubt that. DW Frost DK's would go for 2 handers, possible.
- Retri Paladins with one handers? Doubt that. Prot/Unholy with two hander and a shield, possible.
- Fury Warriors going for 2 x one handers? Possible. Prot Warriors going for two hander and a shield, possible. Arms going for one hander, nah I don't think so.
- Oh, almost forgot Shamans, but you get the idea how it would be.
I had seen plenty of people tanking with a 2H / Shield combo transmogged (on the other server naturally), or DPS'ing with a 1H.
I mean, transmog in general is just personal preference; so it seems pointless to argue about whether people might - or might not.
As it stands, there are a lot of 2H items with small models, and 1H with 2H models (Greatsword of Horrid Dreams is one such 1H). So it's not even that much of a debate anyway, it just gives more control over what you might be able to mog.