I have an aversion to too many different batteries. It's very inefficient from a supply chain perspective. To put into context of producing cars, suppose a carmaker had 10 different models of cars. And all 10 had different axles. They have to stock 10 different sets of axles. And they dont know how many of each car they'll need to produce in the future, so due to the time constraint in having axles made, they have to slightly overstock 10 different axles. Or they risk not having the axles to make whichever model of the car turns out to be demanded.
Now imagine the same car company had 10 different cars that all shared the same axle. They dont need to overstock as many axles because if one model is very popular, and one model is not popular, then they use fewer axles on the less popular model, and more axles on the popular one but they're the same axle.
Back to batteries, suppose you want to make sure you always have a working battery. Two is one and one is none. If you only have one battery it might die and leave you with zero. Suppose you plan to use the tool a lot in a certain day, you might need three batteries to be safe. And what if one or two dies under warranty, now you need four or five while you're waiting for the return.
Or maybe one or two dies outside of warranty and the batteries go on sale for 30% off a few times per year, so you either have to have extra on hand that you buy during the sale or you risk having to go to Home Depot and buying the battery at full price.
So why do I flat out refuse to add a new battery to my lineup? Because I already have a dozen M12 batteries of different sizes. If I got a new platform, now I need to add at least three more of that battery. And it probably comes with some subpar bundled battery as these things usually do, so now I wind up with four of that battery taking up space. And another charger taking up space. And you shouldn't store lithium batteries in un-airconditioned spaces so they are taking up living space.
That said, from all reviews, the M12 dremels are junk and I have avoided buying one. I want one, but I don't need one, and the few times per year I need a rotary tool, I bust out my Foredom and use that. It's a lot less convenient for smaller jobs than a cordless Dremel would be, but I flat out refuse to add another brands battery to my toolset for one tool. And both the old and the new M12 dremels are reportedly junk.
I was considering getting an M12 straight die grinder with a step down attachment for smaller Dremel size bits, but I was holding out hope on a new brushless m12 Dremel. Which finally came around 6 months ago and is also reportedly ****.