Then why do you always see sniper IN M3 early game? Because:
Sniper in M3 is more mobile
You can backpedal in m3 while still shooting. If you dont have m3s you have to retrat much earlier
Not really.
M3s are fail safes for med skilled players.
You will never find top 200 players going with dual sniper and dual M3, that doesn't work.
You will either see a single sniper in M3(which is a waste, because you have 440mp/20fu doing exactly what 360mp can do alone if you have half decent micro and conscripts before you) or dual snipers on foot that are vulnerable to long flanks.
You're really exaggerating by saying that you always see snipers in M3s.
Its just as popular as snipers on foot with better players not using M3s for snipers, but for flame CEs, which allow much more offensive use and actually allow M3 to pay for itself.
The better your micro is, the less you will rely on M3s and more on your own positioning.
I'm not super skilled, but even I don't use M3s anymore, well, not for snipers anyway.