While many carriers own their own towers, there are many independent companies that actually own the towers themselves. They rent the space on the tower, as well as on the ground to the individual carriers. I have worked this industry in the past - and have been at sites that have multiple carriers all at one location.

I personally try to use the packages when I can. It makes it a bit easier for myself to keep track of everything.

It's all in what you need to do. If you need those optimizations or special build options that aren't in the package, go ahead, it's what it's there for.