if apt-pinning is a tricky job in proper debian and if misused can cause serious troubles is even more complex in lmde mainly because it already ships a preferences file and tbh in the end of the day it causes more issues than it worth. (1)
(1)- let's be clear about this: the preferences file shipped with lmde works and works well in the closed ecosystem of the UP. period. but even there if you decide to compile and/or get "alien" packages (as in from outside the default repos) soon you'll face barriers and errors that are easily solved adjusting the preferences like here
is a problem when you for example add the experimental repo to lmde; according to this
and may i add, my experience with proper debian as well, if/when you add experimental nothing happens (as it shouldn't), in lmde (because of that rule) you get a system upgrade to experimental and most certainly a broken system.
on the other hand the 2 first rules in the preferences file are responsible for some of the mint-specific additions and tweaks (a couple app, fonts configs and rendering); if you just get rid of that preferences file, the first thing (and probably the only really annoying) you'll notice is a poor font rendering, but you are also seeing that is harder and harder to keep the ubuntu-patched libfontconf.
here you see an example
of apt-pinning trying to work (to a certain extent) with the default lmde files.
here you see the other side