Its also worth mentioning that adapters are available to convert between battery systems. If you're on Milwaukee and want to buy a DeWalt palm router (which is superior IMO) then you can just get a converter to use it with a Milwaukee battery. You can keep the converter in the tool itself, and most tools don't mind this.

The exception is Ryobi. Converters only exist one-way, since Ryobi still uses "stick" type batteries for low voltage stuff. The opposite converter could theoretically exist (say, to use a Ryobi battery with a DeWalt router) but it would be very large and bulky and so nobody really makes them.