Thats a much stronger reaction than I've heard generally. He's not claimed to have proved his hypothesis, he's claimed to have disproved an opposing one. That doesn't sound like a bad approach to me, people are lining up to disprove his hypothesis! The paper I read was worded very conservatively, and concluded that the findings

did not exclude the original (biological) hypothesis.

Hardly swinging from the rafters shuting 'I've found martians'!

And I struggle to get from your first five paragraphs where Mackay has destroyed his reputation and will never be taken seriously again, to the last where science is not blind and it's important he continue to publish!
No disrespect intended Nick but that reads more like an angry knee jerk than a considered reaction. Even if he is getting laughed at in conferences (which sounds to me like an nasty audience needing a sharp lesson in good manners, rather than a reason to doubt his work) the paper published in an independant peer reviewed journal ( here ) deserves to be taken seriously, either that or the journal needs investigating!

I know there is a NASA published paper, and yes it seems to have more provocative SEM pictures of features that bear a resembalance to established microfossils than scientific depth, but given NASA's current predicament I'm not at all surprised by that. If thats the one you've read then I understand your reaction, but frankly thats what I expected of NASA at the moment.
The paper I read in 'Geochimica et Cosmochimica Acta' was much less sensationalist, and certainly not worthy of such a negative reaction, unless you have doubts about the impartiality or quality of their peer reviewing.