I should have said …
Suppressing fire and Overwatch are the only reasons to take a full auto weapon with the switched DoS. But you will never hit anything, just pin. Both Overwatch and Suppressing fire impose a -20 BS penalty when used and combined with the one hit for two successes leads to an almost impossible chance to hit. Firing in FA will only ever be able to hit 1-2 times a round for most players while expending twice the ammo of a SA burst which with the same roll will hit with almost all rounds.
You will need to hit by at least 40 points under your BS to hit 3 times with a FA using your switched DoS.
You are going to relegate your guardsman, most likely to wield a FA weapon of any note, to go sit over in the corner and waste ammo while the people who can hit go take care of everything. Even better just get the Adept in your party, most likely to have a low BS, a Heavy Stubber. Sit him in the corner of the room doing nothing but suppressing fire. Doesn’t matter if he has any chance of hitting at all or the skill to use the weapon, everyone else can sit back and take everyone out with SA bursts while they are pinned.
Basically I think it is a very bad idea and makes FA weapons a poor choice at best .