One of my friends always buys kits, says she doesn't know how to pick fabric. Its not so, as the times she has picked her own fabric for a project, it is beautiful. She just likes having everything she needs in a package.
I rarely use a kit, because picking the fabric is the part I love the best. Its a personal preference. If you enjoy doing kits, do it. If not--pick your own fabrics. Both ways are perfectly correct. No quilt police here. grin