I've been cross stitching now for about 15 years and have nearly always, although not exclusively, used kits. When I began, I wanted to stitch but needed a kit with full instuctions in order to get started. I have since done some small projects to my own 'design' although I don't see myself as a designer. The advantages for me are that I can get straight on with stitching and some of the designs available are amazing.
The number of kits done in a year varies according to the size of the project and the time available (I also knit a lot and make quilts for family members -strangely,I've never used a pattern for a quilt, preferring to make it up according to the dictates of the child for whom it is intended and also of the fabrics chosen). For example, some 10 years ago I cross stitched an Eva Rosendstand sea and ships scape, using a kit; it took 18 months to complete and was the only stitching project done in that period - it is now framed and is often mistaken by visitors for a painting, so superb is the design which I could never have done. Last year I did a lot of cards, but fewer bigger projects and the total was probably around 20 kits but many of them small.
I now have quite a collection of threads and of fabrics and would probably, in future, restrict my buying to charts rather than complete kits.
I've recently taken up crewel work and have begun with two kits - again for ease of getting into a new embroidery style but can see myself designing to some extent - using historical sources - in future. I've also done some small canvas work pieces without kits.
Disadvantages of kits; there are not always sufficient threads supplied and I often choose to stitch on a different fabric to that supplied.
Which is preferred? Depends on my mood and time of day. I am fascinated by free embroidery but, if I'm looking to stitch in the evening in front of the TV I'll cross stitch because I know how to do it and can do do without too much intellectual effort.
Hope this is helpful.