Have to chip in here. 
IMO, SMG soldier is a good choice in at least one circumstance.
You’re on a pub server with 30+ players. Combat is waging deep into both teams territorries -> there is almost no safe spot in the relevant parts of the map.
You want to get up to an important position (eg side entrance on radar with commandpost not yet built, or where the old city definitely ends in oasis for allies) and do some serious damage/ defending of that spot (heavy weapons)
You’re not yet lvl4 soldier (smg+heavy w) so to get safely to the front you need to pack your trusted smg, and since there is so much ppl on the server you can be sure there is heavy weapons lying around at the front. (or even: the one you used before you got wasted )
Then you can switch heavy w as battle wages and you need to go from eg mg to panzer if you’re attacking. Though once that defence choke is down you want to advance and to do that safely, pick up that stray smg on the ground, defend yourself as a part of the advancing team and then at the next scene pick up whatever heavy w you can find/ is useful.
my point -> the flexibility is the strength, a heavy w soldier without a smg is a sitting duck for an experienced opponent.
You can use this to your aggressive advantage: be a smg soldier in a tunnel, waste the front panzer/ flamer before he gets you (not very hard even if you spot each other at the same time), he drops his heavy w, pick it up and waste/push back his pals (which are already backing off), pick up your smg again and you’re ready for one-to-one combat.
IMO soldiers are (or at least should be) the ones pushing the front forward, clearing mg towers, making certain areas no-go for the other team, to move safely between such positions, having a smg is a great relief.