You are right about startup times, it's only slightly longer with the card.
I don't follow your point about changing maps in internal memory though? If you plug the Nuvi into your computer, it behaves exactly the same as the memory card.
Also, the newest Nuvi series - 1200/1300/1400 do not have any map filename restrictions. On these units, you can create a \Map directory at the root of the internal memory filesystem (probably also on a card, but I haven't tried). Now drop any map files you want into that directory. As long as they have an .img extension, the Nuvi will recognize them. You can name them whatever you like, however when you choose maps on the Nuvi you won't see those names. The name assigned to the mapsets themselves is what will be shown.
But if you put .img files in the \Garmin directory (as you would do on other models), they are ignored! It will not even recognize gmapsupp.img in the \Garmin directory. On a memory card, it DOES recognize gmapsupp.img in the Garmin directory however - weird.
On the Oregon/Dakota/Colorado there are no map filename restrictions either. However, on these models you need to put your files in the \Garmin directory (internal memory or a card). Call the files whatever you like, just be sure they have the .img extension.
I'm curious - do either of these techniques work on your Nuvi 500? Are you running the newest firmware? I though I'd read that it was like the Oregon and would recognize any .img file in the \Garmin directory.