Optimum efficiency in the game isn't necessarily the goal, though. As a player trying to win a game it's certainly nice, but I don't think it's something they would be as concerned about in-universe. There would be other, more important factors.
A 55 ton mech at 5/8/5 will have a 1.5 ton advantage over a 60 ton mech. "Great, let's do it!" you say. But perhaps you've got a good contract for some 300 rated engines, while 275s are more expensive or harder to get. Better to have an operational 60 ton mech that has a little less weight to play with, rather than a perfectly optimized 55 that is a hangar queen because the engine parts aren't available. Same thing with the heavier jump jets. Maybe you've got a bunch of 1 ton jump jets sitting around, or your defense industry needs to maintain the ability to produce those jets, so you need a mech that uses them.
As far as the rear-facing medium lasers, that might be quite useful in warfare, even if it isn't reflected in our tabletop experience. Let's suppose for a minute that the Quickdraw gets assigned as a light mech hunter. Now in the game, players tend to avoid designs like Wasps and Stingers. But we know that background-wise the Inner Sphere forces contain a significant number of these. What is the best way to fight a heavy mech if you're stuck with bugs? Easy, get behind them and swarm them. If you know the Quickdraw is likely to be assigned to go hunt bugs, and you know their primary tactic is to try and get in the rear arc, then giving him good rear weaponry makes a lot of sense. The Quickdraw actually has really good rear armor for its weight class. This sort of mech might be really useful to a battlefield commander, even if it isn't a player's preferred type of game to play.
When I see a mech that isn't so great, I ask myself "why would somebody have built this?" If I can think of a good reason, I just accept it and move on.