And as noted the design is a bit screwy - CHECKSIG feels like something that was refactored out of a pre-existing code base rather than something you'd actually sit down and design on paper.
Yeah, a more apt name would be OP_DO_BITCOIN or something, since it rolls so much core functionality into one inflexible "opcode." The rest of Script seems like pointless toys, since not even the most trivial inspections of the transaction or blockchain is possible. It is cool to see the hash collision bounties that have been published using some of the other Script opcodes. (I'd be interested to see other novel uses, too. Maybe smarter people have found that Script is not as useless as I think.)