I'm in the top 10-50 in 2v2, 3v3 and 4v4 automatch (slumped a bit this week I assume, been pulling 16 hour work days, not had time to play).
I've never seen it.
As you said. Players build the tanks because they need the tanks. We don't have time to twiddle our thumbs in base and wait for a push in a bit.
The closest I get is when I have jacksons or wolverines in 2v2. Maps are small enough that I can just have the crew wait by the vehicle having a smoke until I know where my anti-armour is needed next. Saves me a little MP over time.
You did not get it...
You exit your 2 tanks and you call 2 more call-in rifleman or you reinforce 3 squads then you get back in the tanks,,, you may also get one more call in tanks... you have virtually no pop cap as US.