The non-ditchable weight argument comes from the idea of a "balanced rig." In a perfect world, the diver would be weighted to be neutral without his/her rig on, and the rig would be weighted to be neutral with near empty tank(s). In that configuration, even if you had a total BCD failure at the start of the dive, you should be able to swim up the weight of the air in the tanks plus the negataive buoyancy from wetsuit compression. Most of those who dive without ditchable weight and can't achieve that swimmable degree of negative buoyancy at the start of the dive (big doubles, thick wetsuit compression, etc.) typically will have alternate means of buoyancy (Drysuit, double bladder wing, even a DSMB can be used as alternate buoyancy).
Hey thanks for these very insightful responses! Yeah I guess I feel like I'm starting over if/when I go BPW. I have my weight dialed in with a 3mm and SP Hydros pro, but now that I'm starting to drysuit dive with steel hp, and the potential of carrying a pony on some dives I have no idea where to begin. I think I'll get the SS for my cold water dry suit dives and get an aluminum in the future before I head back to the Caribbean.That being said, many divers don't dive with easily ditchable weight, and consider ditchable weight to be hazardous. The thinking here is that ditching weight at depth can cause an uncontrollable ascent, which is a very dangerous situation, especially if you are diving under a hard ceiling. And sometimes the weight will end up being ditched on accident, if the quick disconnect is not plugged in all the way, gets snagged, or something like that. In the event that you would want to intentionally ditch weight, most folks would agree that it should only be done once you reach the surface, and at the surface you could ditch your entire rig if the non-ditchable weight is a problem.