There are uses for both the LL and the LPL, and each excells at one thing in particular.
The Wraith is sufficient justification by itself to prove that the LPL has a place in the arsenal, although I consider it a "specialty" weapon. Generally, I'll take a LL over the LPL, except where either the firer or the target is expected to generate high movement modifiers and move into point-blank range.
So far, the discussion has been between a 5T weapon and a 7T weapon, rather than about a choice between a 7T weapon and 7T of equipment including the 5T weapon. If that's NOT the real issue, then the discussion also needs to include all of those cases where 7T of primary weapon is simply not practical.
If you're using a LL, a MPL is useful to provide backup firepower against high TN targets, while a LPL with the same tonnage as both covers everything within spitting distance roughly as well, but nothing beyond 10 hexes. The LL can hit out to 15, and will have the same to-hit odds as the LPL at 4 and 7-10 hexes. The secondary MPL will double-cover the 1, 2, and 4 hex ranges with an additional 2/3 the damage of the LPL and identical to-hit numbers, so the LPL clearly beats the LL/MPL combination ONLY at a range of 3, and offers NO shot at 11-15 hexes. The LL/MPL combo also allows you to choose between firing one, the other, or both, depending on range, to-hit odds, and heat concerns.