I was under the impression that P2SH was intended for all multi-sig transactions, though you are right there is no reason plain-text multi-sig can't be used (is there? they were probably never added to isStandard...). One reason for desiring P2SH, in general, is that very long scripts become part of the TxIns instead of the TxOuts. This means that they will never bloat a pruned version of the blockchain. If everyone only used P2SH, then no TxOut script would ever be longer than 22 bytes.