My knowledge of Monster is a little dated.
But a couple of years ago they started the "Catalog Generator".
The "Catalog Generator" produces a set of "seo friendly" pages, separate from the "store" pages. These "Catalog" pages had flatten urls without dynamic strings.
At that time, there was a seperate change to have Catalog pages, and you were charged on a sliding scale by the frequency of regenerating these secondary pages.
I worked with a several store owners that opened monster sites in addition to their yahoo sites. They all dropped the monsterc sites after awhile.
However, these owners already had successful sites and they were just testing alternatives. Most of them they just didn't have the time to manage and promote a second site, and learning a new platform just erased any benefits monsterc offered.
The things that attracted ystore owner's to MonsterC was no rev share and the promise of a higher level of support.
The downside, was the paying for a patch for search engine rankings and the learning curve associated with the platform. The lengthy check out process was also a negative.
Once again, my knowledge is somewhat historic, so I welcome anyone with current info to chime in.