At the risk of being accused of going off topic I'd just like to respond to that by pointing out that this is why I hate internet shopping. There is no personal involvement, no feel good factor about making a decision based on anything other than price, It just so demeans the experience of acquiring equipment. I like to talk to the people I am spending money with not just spend hours browsing the web to find something a few cents cheaper from supplier X rather than supplier Y. A market that is maintained and manipulated by cold computing is one that I'd rather not get involved in but I guess there are enough people who think it's clever out there to keep the momentum of this development rolling. Can't say I'm one of them.
It depends on the product and its use. A Canon/Nikon/Fuji just works (mostly) out of the box. For stills, they use their own accessories which are designed to work together. The high quality thrid-party lenses have seamless integration. I've mostly purchased gear for business, so the joy of aquisition isn't a priority for me. Saving money was a priority.
This isn't necessarily the case with MFDB. An MFDB can be used in many ways, with lots of third-party cameras/lenses, old cameras, new cameras, etc. There are software issues, mechanical problems with older cameras, power problems with newer laptops and tech cameras, and there is the cost. I'm not damning the backs at all, its just the nature of the things. These issues are helped with a dealer who can walk you through it before and after purchase.
At the end of the day, I'd buy a Phase or Leaf back from a dealer, new or used. If I wanted an H or a Pentax, I'd be fine with Best Buy, because I would only be using an H or a Pentax body and lenses. Cuts down on the variables which can cause issues where a dealer would be useful. If I shot a tech camera I'd go the dealer route, or had any "unusual" use where extensive testing is required.