There was a very fleshed out post along these lines, about a month ago. The gist of it was, perks should be inherent to the "ghost" and not the shell. The shell should just be a cosmetic item, not a skill tree. And, I tend to agree. For example, I'd like to use Sagira's shell, or any of the other random little shells I've acquired - but they're pretty much useless. As you already stated, I run a Guiding Light shell, because not doing so puts me at a disadvantage and the other perks are pretty lackluster comparatively.