Sure, neither of them are a big deal, but it's more stuff to keep track of. Much easier to grab a couple 9v's at whatever store happens to be around the corner, and they fit without needing an adapter. I agree that having the ability to use multiple power sources is great, but there's the trade-off of utility vs. ease of sourcing components when you really need it. It's also much easier to get second stage guts pretty much anywhere if you're diving. And quite a few ADV's only use internal components, so they're moderately brand neutral. "If I fits I sits" as it were. I've been several places where there's more than one dive shop, but not an auto parts store in miles, let alone maybe even on the same island.
Like I said before, it's not really a big deal, Meg's are as close to bombproof as a rebreather gets, it's just something else to think about and bring along. If my spares kit can be nothing but a cell and a few o-rings that's less to keep track of than a bunch of specific components. I definitely agree with the Predator thing though, although I have a couple 14500's I use instead. When it dies it'll get swapped with a Petrel and then it'll use anything. One of the reasons I like AA and C-cell backup lights is because it's two fewer sets of rechargeables I've gotta bring and stay on top of when I'm traveling. If I blow up an 18650 there's no guarantee I'll be able to find another one. Again, it's a trade-off between bringing enough spares to not have to worry about sourcing stuff in an austere environment, and having commonly available stuff that you can find anywhere. Ultimately, you do what you have to do to dive.
In the context of the OP, speculating on the reasons for popularity and weight, if the goal is something light and easily packable to travel, then unit popularity might go out the window. If the goal is easy sourcing of parts and ready access to maintenance, picking up the lightest unit might not be possible. If the travel aspect is most important, it may be preferable to choose a unit with the most commonly available (and non-rebreather specific) parts to ease the potential maintenance issue.
Either way I don't think any unit is popular enough to have a random dive shop stock a bunch of parts, outside a few certain places where there is a large following of a particular unit (e.g. APD units in the UK). However, rebreather hot spots will be more likely to have access to several units parts supplies, PDC/Tulum, Northern Florida, etc. If one wants to frequent those locations, certain issues will be far lower on the list of requirements when choosing a unit.
