Partial Wing rules were considered during the Quick-Start Rules discussion, and even partially incorporated in the current rules, but not quite the way you're thinking. This was a decision that was made after we'd settled on WiGE movement for AirMech mode, BTW.
One of the concerns everyone had was how to encourage the use of modes other than AirMech mode. I mean, a typical trimodal LAM has all three modes, so they have to offer some advantage or utility, right?
To that end, one of the things I proposed was that LAMs in BattleMech mode should get benefits equivalent to having a Partial Wing: ie. a +2 to their jump in 'Mech mode and an additional 3 points of cooling. The general consensus, though, was that because a Partial Wing is 7% of a 'Mech's mass and a full LAM conversion is 10% of a 'Mech's mass, but also gives you AirMech and Fighter modes, that giving them all the benefits of a Partial Wing was a step too far. Instead, we settled on the benefits to heat dissipation, since the wings are going to increase surface area no matter where they are, but may not be in the most opportune spot to provide lift in BattleMech mode.
Well, the few times we actually put LAMs on the field (beyond something super abstract like a scenario intro of "an allied LAM recon squad discovered this force, go hit it or something"), we never really used 'Mech mode except to store idle units. For actual combat it was AirMech mode, with sometimes switching to ASF mode to get around somewhere real quick if the sky was clear enough.
IMO, that's probably because there's a significant opportunity cost to making a 'Mech an AirMech. If you're bringing that instead of a regular 'Mech or even a Partial Wing 'Mech, you want it to do AirMech things. If you're using it to do Battlemech things then you just have a lousy overengineered Battlemech so why didn't you bring a regular one in the first place?
Plus, a lot of the canonical Airmechs don't really get much use of that +3 heat dissipation? A Wasp LAM can already jump and alpha strike all day. A P-Hawk LAM can alpha strike with minimal heat buildup, but it's honestly better off doing hit and run attacks in AirMech mode than it is jumping around in 'Mech mode.
I can see the logic on paper in omitting it due to the 5% (3% for IS) weight difference in partial wing weights vs. LAM conversion gear, but given the rest of the restrictions that exists (no crit splitting, no XLEs, no endo structures, no fancy armors, no omni), I don't think it panned out that way in practice. And while I am talking about my games, I also mean in the canonical universe.
Every canonical Battlemech with the Partial Wing system (either IS or Clan) uses some sort of advanced component that LAMs can't use (most commonly XLEs, though the Peregrine 7 opts for endo and FF instead).
Basically, you lose the option of having any of the other neat trinkets, knick-nacks and doo-dads to become a LAM. The fact that (in canon at least) your 'Mech has only +3% dead weight compared to "Mech Z," a theoretical IS 'Mech with a partial wing but no other fancy trinkets, doesn't actually matter, because Mech Z simply doesn't exist in the universe.
Of course that's just a sample size of one person, but that's my 0.02 C-Bills.
