Someone once referred to it as the "magical murder stick" when applied to infantry and vehicles, and that's always stuck with me.
Here's another good example: Anywhere you have a standard AC/10, try installing a Plasma rifle instead. Same range, same damage, plus it adds heat. And as a special bonus, its ammunition won't explode. And installing it will free up 6 tons of weight for other stuff.
As an aside, someone mentioned the additional heat really messing with aerospace units, and this got me thinking: Does any weapon inflict heat to Capital scale targets like WarShips? I mean I suppose I could drop huge amounts of XCT Flamer infantry onto a WarShip hull, but that seems a bit dramatic.