Aramis had the 2nd best OPS+ of his career in 2012. Not showing signs of a decline yet. The guaranteed portion of his contract only runs through 2014, with an average of $12 per season in the deal. I don't see that as a bad contract for the Brewers.
Basically, Cubs swapped Aramis for Pierce Johnson. So, that might turn out great in the long run for the Cubs.
I think letting Aramis depart had a whole lot to do with culture. I think Theo has implied as much in some of his general comments about unspecified guys who are no longer here. Theo didn't want him--that's my sense.
There is no getting around the dramatic turnaround in 3B production caused (so far) by his departure and the dramatic improvement for the Brewers. As I noted a few days ago, Cubs and Brewers basically swapped spots from the top to the bottom in 3B production from 2011 to 2012. Incredible offensive difference.
You can say that it doesn't matter because Cubs are non-contenders and that having Aramis around might well have pushed the Cubs draft spot back from #2 this June, etc. But, no getting around that current Cubs lineup would be dramatically better with Aramis at clean-up instead of Valbuena at #7.