I figured I would throw my 2 cents in here..
First off, for decades when you draw out a network infrastructure on paper or with visio or something, the internet was always depicted as a cloud. This has been "industry standard" since the internet has been a part of business. The term became commercialized a couple years ago to mask that it is not any kind of new technology. I think of the "cloud" as a joke.. an example of successfully relabeling and marketing to the ignorant.
Second, the "cloud" as a medium for storage has some benefits to business and to a lesser degree, home users. When you want to share files across multiple locations, multiple "outside" employees (ie road warriors), and keep ONE master file of anything, and keep it secure, a "cloud" based solution can be great. Not every company can afford to have secure access for employees using public internet (ie random IP addresses) back to their own servers. And even big businesses should have some level of geographical redundancy with their data, so a "cloud" based backup at least on a weekly basis is prudent.. The modern "cloud" services fill these niches for businesses. For home users, being able to access your files from your mobile devices securely can be tricky. Most off the shelf devices have adequate security, but people don't usually have a network engineer setting things up at their house. It is all too easy to leave your network very vulnerable. At least with web based "cloud" services people aren't messing with opening ports in their home network to the public internet...
Third, I can understand the idea of having one working set of roms and CHD's for each version of Mame that anyone could connect to and use, but between the copyright infringement, performance issues, and just the fact that you would have to convince people who are getting roms for free already to pay for a service to not even own them (and have to put up with performance issues), it just wouldn't work out..
I CAN see how if this could be a HUGE benefit to me IF it could work, which I don't think it could in any realistic capacity. So the question is a legitimate one. I have my mame machine, one at my brother's house, and soon one at my mom's house. In the next few months I will have 3 more out there at various friend's houses. As time goes on I improve on my files. I refine control panel graphics, fill in missing artwork and videos, update the controls.ini and colors.ini (for LEDblinky), tweak the mame.xml file, and even recompile mame to add new features I hadn't messed with before. If I want to add those changes to other machines, I have to go to each and update them manually. It would be pretty cool to have them all running off a single source in "the cloud" and have them all work off the same set of roms and other data.
As for just using it in your own home, throw out the term "cloud" and ask about using a NAS device (or just file sharing with a basic server in your home network) to host all your files for multiple mame machines. This I can see working. Sure it would be a little slower to load, but nothing major at all, and you wouldn't have to replicate all changes to each machine. But the economy of it makes little sense unless you personally have a LOT of machines in one location. Maybe if you already had a media server set up in your home and wanted to be able to access the roms and CHD's from all your devices (HTPC, mame cab, gaming computer, tablets, smartphones, etc) so you can game from anywhere in the house on any device. But in reality even that is just a case of doing it because you can, not because there is some kind of benefit..
I will take this one step further though. Add this idea just for hiscore files and apply it to Yotsuya's digital high score board. Imagine if we all used a "cloud" based hiscore file and could access it through a web portal that we could display on any computer or TV.. Take it a step further and have mame access a shared, cloud based hi score file for each game so that we have a high score list on certain games that is based on the whole community, real time and live. THAT would be cool.. Never going to happen, but it would be cool.