Well sure, choice items are fine and all but then you realise what choice users use: Aggron uses Head Smash, Rotom-C uses HP Fire. In fact, most choice users have some kind of coverage Shedinja is weak to which, with some good prediction, can leave you one Pokemon down. Plus, you said it yourself, Shedinja is situational. This isn't a set which can be situational, this is a whole slot on your team that only works against certain kinds of certain Pokemon. I get that he can work, but then you have to acknowledge that the rest of the time it doesn't. And by the rest of the time I would give 90% of the time at least.