Looking for a rear arc when only 1/4 of the weapons are aimed in that direction is kinda lame. First off it would require additional rules which could be confusing. A rear arc at the same attack value isn't that bad. A 360 arc isn't bad. Having multiple firing arcs with different attack values is difficult to convey.
Heck, lets just play it mathmatically and say that 1/4 of 3 is .75….so really it does have a rear arc but it's attack value is .75 of a die, and since we don't have the appropriate die, we can't use it. Or wait, maybe they can give us a new 6 sided attack die just for the lambda shuttle's rear arc. It could have a hit, an eye, and 4 misses.
If you want to get this technical then why aren't we fighting for different firing arcs on other ships? An Xwing's weapons are 'fixed' which means you have to line the fighter up directly on target at the right distance to actually hit with all or any of the weapons. However an A-wing has tracking laser cannons that have a wider firing arc. It should really have 3 arcs. 1 main arc that is represented on the current bases, but then it should have two wider side arcs that have less attack value (where only 1 laser can reach). If it flew on it's side (compaired to the other models) it would have a full attack 60 degree forward arc! Some even could fire behind them, so really if they were the advanced models and flew on their side they'd have a 360 degree arc! The YT1300 (the falcon at least) had additional weaponry that tracked also (the small retractable laser cannon). The standard lambda shuttle has some fixed forward lasers and some tracking mounts, so it should have a standard firing arc, a rear arc, and a wider secondary forward arc.
Imagine the base of a LAAT. There'd be at least 5 attack arcs! Yes, yes, thats not OT, but I think you get the point I'm trying to make. In games, we have to suspend disbelief sometimes to allow the game to flow and play easily. The no rear arc of the lambda is just another minor foot note in a long list of modifications made by FFG to make the game fun and quick instead of a complicated mess of miniature rules.