The operative phrase here being "for me". For you Spotify is perfect, for many others like myself it's useless. I listen to a lot of music that simply isn't available on Spotify and never will be. I also live in an area that has spotty cell coverage and I spend a lot of time in the car. Not only would I never want to waste my 4G on streaming while in the car, I couldn't even if I wanted to.
Your first point is certainly a limiting factor, though it's worth noting that you can also use Spotify to play local tracks. However, for your second point, Spotify* enables you to cache playlists offline, in part to avoid the very problem you mentioned. And if you're driving, I certainly hope that if you're driving you already have a playlist made and aren't searching for new music at the same time. So as long as you don't bump up against the cache limit, it could work for other people with this problem--but of course, for you, the first is much more limiting.
*Actually, Spotify Premium, but you need this for mobile anyway.