Where I see Plasma as kinda viable outside of my described Ambush scenario is with Apothecaries, Tech-Marines and Librariens. The Tactical Marine gets free special ammo and potential bonus to damage and toHit with a Bolter, he won't change to plasma. The Devastator and the Heavy Bolter are talked about in great detail in many threads here, so lets just say, no reason to switch to plasma.
Generally lower BS value in these specialities, and really usefull Skills to use during recharge. So for example, 1st Round : Shoot the bad guys with maximal Setting, 2nd Round: First Aid, restore 11 - 17 wounds to your Devastator who draws fire like nobody else.
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