I'd like to go in that direction too - have you decided on an approach?
I'd like to not have certain programs get pulled out from under though (I have this vague impression that sometimes things leave testing temporarily) -- do you think that could be a potential problem?
It's a distinct possibility although I didn't encounter much of it previously. Also, whenever something did go seriously wonky following an upgrade, it was usually fixed within a few days.
The biggest difference I've seen with LMDE is that it isn't quite as polished as the Ubu-based versions. (And to Ubuntu's credit they do add a great deal to enhance the entire enduser experience. Something you won't always notice until you try something else.) So LMDE sometimes needs a little more tweaking to get everything "just right." That's in contrast to "standard" Mint which needs virtually none.
Previous LMDEs were also noticeably faster than the Ubu-based versions. I don't know if it's still the case, but I'm guessing it probably is. On a fast enough machine it probably won't matter. Regular Mint (on a modern PC with around 4Gb RAM) is plenty fast anyway.
I guess the best way to think about it is to remember you're running Debian
rather than Ubuntu
with LMDE - with all that implies. If you let that understanding guide your decision and expectations there shouldn't be any bad surprises. If you've had previous experience with Debian you should be fine. If not, you'll possibly need to learn a little more. But learning something new is always a good thing, so no knock there.
Addenda: regarding approach...
I have the luxury of having several machines at my disposal. So what I do (and would do even if I only had one machine) is throw a new drive into whatever I'm installing on and go from there. I'll keep a new distro up for a week or two, to see how well it works for me, and decide if I like it. (Just because it works doesn't mean I'll want to spend my workday using it. These days I expect more than "just works" from a distro.)
If it makes it past a few weeks with no major disasters or showstoppers - and find myself using the new distro more and more - it's a keeper. If I really
love it, I'll install it on everything. If I just like
it, I'll keep it on whatever it's running on and leave the other machines as is. (FWIW I'll usually have two or three distros running on my network at any point in time.) Over time. and with day to day use. there's usually one distro I find myself obviously gravitating towards. That becomes my go-to distro until something I "really
like" more comes along. It's kind of an organic approach to distro selection and deployment. But it works for me. YMMV.