I think what the LFS meant was the types of fish will be limiting if a reef is kept. FOr example, Butterflies and large angels, triggers etc are not reef safe so this means they are not an option to be keptin the tank.
As for the MM system. Im not sure how to take the comment, my fish dont "survive" they are in great condition, and with some of the Corals i have (mostly Acros) these are quite delicate and need an excellant filtration system.
MM is not a crazy idea, its tried and tested and what is even better its a totaly natural way to filtrate your tank. The mud gives off mineral and trace eliments that will allow macro algea to flourish in your sump. These algeas i n return will doa fantastic job at removing NItrates and phosphates. I have a huge poluation of pods in my sump along with all sorts of other life, brittle stars, bristleworms, cerith snails and stomatella snails etc. This ecsystem is running very smoothly indeed. With the addition of mangroves this will help nutrient extraction even further.
Mechanical filtration is ok and does have its place but my view is that nature has had roughly 200,000,000 years more experience than us and our mecahnical filters are very poor in comparison to liverock and macro algeas.
As my tank is mainly an sps tank with the majority of them being Acros. Its important to run it nutrient poor. They dont like night nitrates or phosphates so if i had doubts that the system would not do its job then i would look for something else.
Edit: The reason i dont run a skimmer is beacuse the manufactureres dont recomend one to be used. A skimmer is another device where man just isnt skilled enough to design smething that can remove harmful toxins and leave the good trace eliments. A skimmer doesnt differ betwen good and bad substance and thus will remove them as well. A real good skimmer will do more harm to a MM than good as it will remove the very tracve eliments that the MM is designed to give to the marco algeas.
I also run skimmerless because i use natural seawater and i think its vital that my corals get the plankton and trace eliments that this water will carry.