Bigger gun, more money, more shooting means you need better parts, means you HAVE to upgrade it.
As a 249 gunner:
1) I never go prone unless im on top of a hill
2) It's rare that I sit back and defend
3) It's rare that I sit back
4) I'm typically the one pushing the assault forward
Really depends on your squad tactics. An LMG can be a very static thing, or like in my case, it can be very mobile firepower.
However it's difficult to employ a static 249 without squad support.
My 249's fully upgraded minues a tightbore, shoots DAMN far, and I'm always on the front, and always racking up kills lol