Defense is less important than offense in my book, because there simply are more guys who can play defense and hit .230 then there are guys who can hit 35 HRs. And you aren't going to win many games with three light hitting OFs who can field their position at a high level.
I think ultimately it is an argument over very little. Neither Mallex nor Kemp nor Markakis is really an ideal player for a contending team.
The entire argument is really over whether Mallex was likely to improve his stock in AAA or as a fourth OF and whether the return they got was a good one.
I think they sold at the appropriate time. The return is well beyond what I have an opinion on. The only thing I can say is that apparently there are significant number of scouts, professional baseball writers, and fangraphs contributors, among others, that think it wasn't a bad acquisition. Time will tell.
I tend to like the story of a young player getting more mature, reshaping his body, and having a big spike in performance. But he's a long way from Atlanta.