I use Krita for free, but I'm not a spite artist.
Mostly, paint not having layers or pen pressure is a absolute no for my style. I do use it for editing and resizing since it's so lightweight, but it definitely isn't versatile.
I don't think it's a problem if paint works for you. It doesn't slow down the computer much, and is really easy to work with, but that's because it's a very simple program, and it won't support most art styles. But sprite art is where it can work great, since it doesn't put much emphasis on layers and brushstrokes. I would, however, consider a heavier program just because if you want to put sprites on multiple backgrounds, something with layers will make that waaaay easier.