Sorry guys for my english. There is a possibility for a minor problem to a more serious damage (especially when you keep adding ubuntu' repos) but you have a chance to repair even the worst "problem". See also the warning there: http://sites.google.com/site/mydebiansourceslist/ (a useful link for adding extra repos). My first point is that Lisa is more compatible with ubuntu than with Debian. There are users, running ubuntus with 5, 10 or more ( ) ppa's added. Just imagine them, running LMDE, with the same repos added, plus ppa's for chrome, ff, libre, vlc,gimp etc.. cause they want a latest version of each of those programs -programs already existed and well-maintained upstream. Secondly, some programs offering also and "debian" binaries, not only "ubuntu's". See eg Opera: in such a case, the debian source is the recommended one for a "debian" user.There's probably a big chance that using a ppa will broke the application you are updating from there, but i don't know about damaging the entire system. At least if the application is a high-level app that doesn't tries to install shared libraries (that may break other programs) or touch important system files.
And if it breaks for any reason, you only have to uninstall it and go back to your distro repository.
But my main point is that a user have to set his priorities. Under the circumstances. if someone wants frequent upgrades or upgrades in a more or less known frequency or he/she worries about security issues, then it is a good idea for him to consider the option to point to Testing repos. It's a matter of taste. So, my comment "But if you do not like the idea of running a rolling release, keep the "latest" repos" was only an attempt to emphasize the difference between this choice and the existing situation with UP's -not my opinion for UP's. I am not stating (here) my opinion for UP's
PS1: there is also the alternative to enable testing repos just to upgrade firefox -but the user must be aware of the dependencies.
PS2: "That "default" makes LMDE just a rebranded Debian Testing with preinstalled multimedia codecs. That's nice, but IMHO not enough to justify a switch to LMDE." Seriously, a topic with that subject would be very interesting -cause there are many users using LMDE that way. IMHO, lmde has an easier and faster installer but debian's installer is better, it depends on the user what he prefers more.