I think there are some special cases when the kit is better. Eg: Dim kits - you will never find the same color floss as was in the kit or when the fabric is painted. I have all DMC color, if I love an Anchor kit, cheaper to buy the kit than buy all floss.
Anyway usually I use pattern only. :D
I prefer to get patterns, but if I see a nice kit on sale I get it. They just have so much extra thread and way more fabric then I would normally use if I cut it myself. Most of my sewing become things rather then framed, I do not need the edges so wide. To compensate I figure the bottom left and normally wind up with enough extra to sew a bracelet. :)
I never buy kits anymore. I have found that there can be errors in fabric size provided, or the amount of floss or other fibers included. I think the kit makers throw them together as cheaply as possible, and it shows.