Bombers are really handy for taking out jacks and coms, but ofc they're locked to nubs :/ Rick that is not a very good argument. Although newbies don't even know how to use the units properly anyway. IMO it wouldn't hurt (except for people complaining its "unfair" they don't have access to everything) if we locked everything except the cloakybot factory, mexes, llts, mts, solars, winds and radar for nubs. But this is ridiculous and much better suited to a SP campaign which we don't have atm. So instead we have a half-implemented hack...
Ideally the multiplayer unlocks will be removed completely (except for com upgrades) and instead players can unlock stuff in a singleplayer campaign, and maybe get a "I completed the campaign" medal so even if they're new players and suck, they will know they know what the units do and are good for.
Many people have made good suggestions for locks that still haven't been implemented (roach, tick, blastwing, etc.) but I'm not really sure how helpful locking is for nubs anyway. They will gain xp instantly and buy it thinking its special and use it poorly just as if they had it from the start. The problem comes from not teaching them (see lack of SP campaign above) rather than making it unavailable for a short time.
(sorry for any typos/train of thought ramblings its quite late, going to bed now)