assaulter squads have more soliders and more chance for automatic weapons across all brs
They combo it with that
No vitality perks, 5 soldiers instead of 7, only 1 engineer instead 2…
So many reasons.
I came to that conclusion months ago myself. 
I run two sniper/rifle squads along with three MG squads and one assaulter squad with the level 3 allies. I run two sniper/rifle squads plus several assaulter squads in level 3 Russians, and am in the process of building my two level 5 German sniper/rifle squads. I love it since i have a semi auto to fully auto rifleman that can also snipe when need be, and that comes in handy a lot as I am finding out. My second Allied and Russian squads have a radioman in place of the medic.
cause it is not some advantage… most of the battles are on short-mid range where you can easily see enemy with normal stg so there is no need for scope and it can even be in the way when you are fighting in CQC.
btw if i ever used that squad i would remove scope from stg so it can be short-mid range weapon and get second rifle for actual sniping.
Im curious, why the two primaries instead of ammo pouch?
this particular stg is like 30-40 percent better than assaulter stg tho
idk how. only thing that is better on sniper stg44 is dispersion, otherwise scope is as much of a upside as it is downside. what you gain on mid-long range in accuracy, you are paying with loss of situational awareness and scope is shit in CQC.
not to mention all downsides that adam mentioned.
No Engineer in sniper squads?? 
How do you build rallies
I get a lot of kills from both the scoped and non scoped rifles, and if it were not so, I wouldn’t be building more squads like them. 
Most other squads other then my sniper squads have engineers, including my APC’s so that is no issue. I also have two squads that I use a lot that have one assaulter and the rest is engineers, so building rally points has never been an issue. The biggest issue I have is that 90% of my radiomen are dead 20 seconds from spawning in damn near every battle. 
Actually… we used to do exactly that.
Sniper MKB was added to the game before regular MKB.
you know for these rifles you could tap right click to use the iron sights instead of the sight?
I could do that, but I don’t for these reasons. First, I am used to using the right mouse to aim and zoom in, and with tapping to hold I would keep messing up instinctually by hitting the right mouse thus taking the under scope thing off. 
Second, I don’t like the view, especially in the SVT 38. Neither zoom very well in that mode so the target looks much more distant, and a lot of the field around the iron sites are blocked by the scopes. But the main reason is that I am legally blind in the right eye, so trying in a fast paced game to see under or around the scope with the iron sites is not that easy. I can see in the right eye, but it is always cloudy. However, even with perfect eye sight, I would choose scope off verses tapping and holding peering under or around the scope.
I removed that mechanic - ADS for me needs to be one click, not a “hold”
You could just swap them with each other. You’d still have the option of both, but the primary one would be more convenient for you.
Do people actually run 2 engineers? I’ve always preferred an engi and an AT gunner or radio operator.
because assaulter squad has 7 ppl. squads with different capacity consume same Manpower when respawning, so bigger squad == better overall survivability