There is a (better) alternative to the SDI solo class: The PDIC solo class. It is a bit more extensive and rigorous (as most PDIC classes are compared to PADI, et. al.) and I believe has been out there longer as well. The real focus of any solo class should be self-reliance and safety (redundancy and the ability to self-rescue). Solo certification should be recommended to anyone who dives and has a reasonable amount of diving (couple hundred dives) under the belt, even if the diver has no intention of "solo diving" in their future. Why? Because more often that many would like to think, you really are solo diving even when you did not set out on a dive alone.
For example:
You and your buddy got separated during the dive. You are now both "solo" diving.
You boarded the boat with no buddy you already knew and were "assigned" a buddy who turns out to be a moron, a distracted photographer, or just plain inattentive to your presence. You are basically on your own with that buddy.
You jumped off the boat first and descended on the line. Until your buddy jumps in behind you and gets down to your level, you are solo. Same thing on the way out. When that buddy is on the ladder you are solo.
You are diving in a group of 4 or more people. Often larger groups or clubs all pile in together and don’t take the time to decide who is whose "official buddy" for this dive. Everyone is swimming around near each other, but is anyone really keeping an eye on you in particular? You are solo (in close proximity to others, perhaps, but still solo).
Your buddy runs out of air, gets cold, or for some other reason heads quickly to the surface. You have a deco obligation or just want to ascend at a slow rate and take a safety stop. You are now solo during this part of your dive.