When you have the composition to support and defend it. Giving away a 50 is bad news. Either support it with infantry or get your other 50 to cover it.
Alternativly when you're floating a bit and don't know what to get. MGs are great to lock down cutoff and VPs.
Lastly, when your opponet is spamming volks, and has no indirect. That's the best time, just so long as you're supporting your 50 cals and not expecting them to 1 v 5 the blob of death.
