A nuisance most certainly, if it was a game breaker most of us wouldn’t be hear talking about it, we would be playing another game or something 
I was going to refrain for posting the following as I’m crap at explaining things but DA’s post in another thread prompted me:
So we’ve all can agree that it is possible to identify a class, just that it takes a bit of observation. Would you agree to in order to observe effectively you need to step back momentarily and take in what’s happening in front of your eyes and wait for these give away signs?
So, lets put this into a scenario, Container City first objective. You’re on defense defending (duh) the gate and lets say you spot 6 guys coming out of the main spawn exit. So far they haven’t gave away any tell tale signs of which class they are, you start shooting at will.
As they take damage you spot the medics as they try to give health buffs and/or revives great yeah? Well no because they already have done their job by giving out revives and health, it’s a bit late, your been reactive now rather than proactive, if you knew straight away as they came out you could have started targeting those medics so that they would have either died or use their health buffs on themselves rather than their team mates.
Anyway, you manage to hold them back a short while near their spawn, suddenly 3 guys charge for the gate, erm who to shoot? If you fire indiscriminately you’re going to draw attention to yourself and 3 against 1 isn’t going to last long in your favor. (I assume here rest of your team are spawn camping :p) You hold back a moment to wait for a guy to whip out an explosive charge, bingo we have our relevant target, you start shooting but by now he’s already starting planting the objective, if you’re lucky you will kill him but due to his close proximity to the objective a quick revive and hes back on his feet planting again whilst you have the other 2 raining down bullets on you. Again you are been reactive rather than proactive.
If you had known who the soldier was much earlier you could have lobbed a nade into that group, momentarily disorientating them and took out the solider and gave your team a chance to fall back and help you out.
The differing of opinions on this matter maybe down to differing preferences of play style perhaps?