I think the point was that it is not their main effort. I was under the impression that LMDE was intended as a parallel effort for the sake of giving the Mint team options in case there is a problem with Ubuntu in the same way that there was an imperfect PowerPC build of Windows NT or an Intel build of OS X long before Apple switched.
Regardless of whether I am completely of base about the intentions, when I last tried LMDE a year ago, I found it easy to break during updates. Though it wasn't difficult to fix my mistakes, the ease with which I was able to make them seemed contrary to the spirit of Mint, as though it didn't yet have a much polish as the main efforts. Things definitely could have changed though.
Regardless of whether I am completely of base about the intentions, when I last tried LMDE a year ago, I found it easy to break during updates. Though it wasn't difficult to fix my mistakes, the ease with which I was able to make them seemed contrary to the spirit of Mint, as though it didn't yet have a much polish as the main efforts. Things definitely could have changed though.