What about Soviet Infantry that usually have slots to spare?
You trade potential of double upgrade that requires more cost for a safe upgrade that is good at all ranges and can never be lost unless the entire squad is wiped out.
Assymetrical Balance.
Bar and brens ARE good at all ranges.
Hell, stg at long rangeis barely different from kar..bar has 5 dps points...
Soviet infantry is dirty cheap, 20 mp to reinforce a 6 men squad, and is getting a buff...give them dropped mg34 and you have got the freaking t1000 squad.