"Practical."
Hmmm... if you don't play a class very often, then it would not be practical for you to unlock any of its weapons. For example, I don't play sniper much. Now I wish I had never unlocked the M95. Please think about that before doing anything.
However, sometimes a weapon might be so awesome that it encourages you to try that class. That is what the G36E did for me and the Medic Kit. The G36E, for me, has been a godsend. It is very accurate from long range, and it is pretty effective up close. Even if a support guy goes prone and starts shooting at me, many times I can nail him right in the head with the G36E and either kill him and survive, or at least kill him while I die.
The MG36 is almost like the G36E for me, but not quite. I like it better than the PKM for its better RoF and I FEEL more accurate with it. The PKM probably has the advantage of dealing more damage, but psychologically I like the MG36 better, and hence, I think my performance is better.
If you really like playing Anti-Tank, I think the DA0-12 is a must-have unlock. You shouldn't be trying to fight infantry when you are anti-tank, you should be taking out tanks, APCs, Vodniks and the like. Therefore, the only time you REALLY need a weapon is if someone surprises you as you walk around a corner, or you are in a tightspot advancing on a flag with yer mates. The DA0-12 is useless at distance, but formidable at close range. As opposed to the other AT weapons which are relatively useless at distance and pathetic at close range.
If you really get turned on by Spec Ops, the G36C is a good unlock. Very, very accurate. Seems to me it is just like the G36E, but I like playing medic a lot more than Spec Ops. I think the SCAR is kinda over-rated.
Don't know why anyone feels the urge to upgrade for Assault. The G3 does more damage and can be used to snipe, but the 20 round mag and lack of a GL are a bummer. I feel that the F2000 is kinda over-hyped, too. I actually don't mind the M-203 as USMC, and the AK-XX variants are devastating at close range if you are MEC, PLA.
Jackhammer is a nice Engie unlock because it is automatic. That helps a lot, because if you don't get 'em with the first shot with a pump-action, you're usually toast. Don't know what to think about the MP-7. Seems to go through clips pretty fast, but a little more effective at distance. Again, as an Engineer, you're kinda like AT - you have no business playing grunt.
So, to make a long story short:
1. Don't waste an unlock on a class you rarely play - unless the unlock is so good that it actually improves your performance to the point where you enjoy playing the class/kit. That's what the G36E and MG36 did for me.
2. Unlocks that are perceived to be improvements (in my eyes) over the stock weapon for the kit:
a. DA0-12
b. Jackhammer vs. Pump-Action (I like the MEC shotty)
c. PKM (gotta have it anyway, to get to the MG36)
d. L85Ax - the first medic unlock, ONLY because you gotta have it before you get the G36E
e. G36C for Spec Ops
3. Game changing unlocks (based on my experience):
a. G36E
b. PKM
c. MG36
d. G36C