It's not just that. For one, like I said, it's being able to play console games portably like many have wanted to do for a long time. A lot of SNES/N64/GCN games have only been made portable because of hardware mods of original systems; a dedicated portable system that could play nearly all of them with actual controller buttons has been very slow in coming, and it's something I'd love to get with the polish that Nintendo is known for and not just on a phone.
For another thing, when these rereleases do well, it says that we want other games like them. The positive sales of EarthBound are most likely what prompted Nintendo to release the NES prototype and what could potentially lead to Mother 3 being officially released. Both those games have been online for years, but good games are good no matter when they're released, and I personally like to commercially support good games when they're available.
I know, those are more legitimate reasons to make a purchase. However, we're now going to be asked to make some of those purchases (NES specifically) for the 3rd, 4th, 5th, 6th, 7th or even 8th time. I own Super Mario bros on an NES cart, GBA cart, Wii vc, Wii U vc and 3DS vc, and that doesn't even count the remake releases (or rereleases of remakes in the case of Super Mario All-Stars and Super Mario Bros Deluxe).
I agree that supporting the release or Earthbound may have led to the release of Mother 1, but can you think of a single other virtual console game whose sales figures accomplished anything?
Not to mention that having a game on a storefront is likely to ensure visibility and access than assuming people will go out of their way to get a game they might not know about.
I agree, and that's exactly why these old games should be put into the public domain. Here's an interesting article about how harsh copyright laws have strangled our supply of books since the 20th century.