It is not unreasonable to bring up failed policies of past presidents - and when talking the 2008 election, Bush is effectively a past president - when candidates are being criticized for supporting the same or similar policies. For instance, several times on this board certain failed policies of President Carter have been compared to policies supported by Obama. But Carter is not running, is he? (though technically he COULD....)
And, of course, many policies supported by McCain which bear a resemblance to the failed policies of Bush, are also being criticized. In that, bringing up Bushes failed policies is in no way an illegitimate argument.
Although, IMO, the basis for comparison is often over extended and ignores a lot of McCain's statements and past actions. The fact that McCain is not well liked - and the dislike is more prevalent among the more partisan "bushite" type republicans should tell the extreme lefties their "McSame" label is inaccurate.