I say if one is just what you want and all the fabrics you love, why not? I have over-bought for many quilts costing me twice what it should have. And it can be one of those well, I will put it away until I can find the rest of the right fabrics kind of thing. Which I have two of, right now waiting.
The kits can be tremendous fun because you are not agonizing over the material.
But, yes sometimes I have to do something I create completely from scratch.