I think the Miers appointment was a mistake, but there is one sense in which Miers cannot be compared to Souter.
Souter was an unknown not only to the country, but to Bush Sr himself. He was foisted on Bush by the dastardly Warren Rudman, who said "trust me." (And he, a liberal Republican, could be trusted to hand Bush a liberal judge.)
In this case, Miers is an unknown to the country, but is intimately known by Bush. Bush could make the argument that he knows her and her judicial philosophy much better than he would know the philosophy even of someone we think would be very reliable, like Janice Rogers Brown, Priscilla Owens, etc...
In other words, in some respect, Bush may have viewed himself as specifically NOT repeating his father's mistake with Souter, which was that of nominating someone he didn't know at all...
Yes, he could do so, and I would welcome such a dialogue. As of yet, he's said nothing of the sort.