If clients use bip16/17 before 51% of the miners support it, invalid P2SH transactions can end up in the chain, because old miners will think they are valid.
when 51% of the miners upgrade invalid p2sh transactions will be rejected by minersand will never end up in the block chain- so even if an old client thinks it's valid - it doesnt matter , because there wont be any invalid transactions in the blockchain
I'm starting to get the feeling the whole BIP12, 16 or 17 ordeal is a big PR mess brought about by not stressing enough the most important points of it all. I now have a much clearer picture about what's going on and I think my OP was flawed by my lack of understanding of how the Bitcoin system works.
Thanks everyone for clearing it up for me. I still have a concern about how we would deal with 100% of hostile miners which I believe is a possibility but I now believe even that is survivable.