I dive a compact AL80 with a 1 mil skin with no lead, and recently switched from my heavily padded vest BC to a Kydex plate rig that I've figured has removed a further 3-4 pounds of buoyancy. (On a recent trip to Hawaii I worked myself down to 4 pounds of lead wearing a beat up rental 5 mil and diving a neutral AL80.)
With all this talk about the awesomeness of ditchable weights, I think it's worth pointing out that weight systems are also a point of failure, if the weight pockets that I occasionally find on the ocean floor at dive sites are any indication, which I am cleverly avoiding by not using. And I never have to worry about moving weights around to establish proper trim. And my spine and leg muscles appreciate less weight when I'm doing a ten minute hike to the dive entry point.
Choice of tank, exposure protection, amount of weight, and how much of that weight should be ditchable are all factors which are considered in setting up a "balanced rig." I'm not a DIR diver, but
the DIR crowd has put a lot of thought into this. There have also been many threads here on SB discussing this. Here's
a link to one of them.
Per those links, a balanced rig can be swum up from depth with full tanks and all weights, if you use them.
I have a balanced rig. If I ever move on to doubles, I likely would need to use aluminum tanks and weights to balance it, but we need to distinguish between the "negatively buoyant at the start of a dive and therefore balanced rig" and "uncontrolled descent to certain death rig" that would likely result if I lost my mind and used steel doubles on a stainless steel backplate.
In the event that my BC has failed on the surface AND I can't breath from my regs or snorkel AND swimming back to shore or treading water is not an option, I always carry a DSMB that I can inflate if I just need to bob along.
So I'm giving up the ability to do an emergency buoyant ascent though I can still establish significant positive buoyancy and upward momentum by filling my lungs, assuming that both my regs haven't failed instantly, preventing me from taking the final big breath. On balance, I'm willing to accept the risk. If ever both me and my buddy both go OOG and I can't do an emergency swimming ascent, then I expect all you guys to tsk tsk at the report on A&I.