Well, I used to use GarageBand, but then I bought Cubasis for my iPad. I also use Adobe Revel instead of iPhoto. That might change when Photos comes out though, as long as I can keep them on the computer, but still accessible on the device.
I use iMovie, the problem is that whatever you import into it, is going to be just a copy of the file. So if it's a 2GB movie, there goes another 2GB of space. You really have to manage your phone very well to use it, and probably live with the fact that you always have to have 10GB free.
It's not that iPhoto is useless, it's just people don't think very much of their photos these days. You have a 16GB iPhone, you're not going to dedicate it to getting your photo albums in order.
And most of us who used iPhoto in the beginning most likely upgraded to Aperture when we started getting serious about photography.