Hi Tony, it's a personal thing I suppose, some prefer seperate boots for the reasons mentioned above, ankle supprt, others like myself, choose them because you have a wider range of soles available on wading boots these days.
My first ever chest waders were Barbour with attached boots and studs, but in rubber, great waders but a tad warm in summer.
Next a pair of Daiwa neoprene waders with attached boots, then the fashion for stocking foot waders came about in a big way, and I followed suit.
Both have their good and bad points I suppose, one thing I am sure of, boot foot waders are a damned sight quicker to put on and take off.
I was recently fishing with a friend who had the Vision Extreme boot foots with the zip in the front for the times nature calls.

With me it's getting more often as I age, so I may try a pair in the future, also he was in his third season with them, and had no leaks, and they looked like NEW.
The waterproof zips seem to be holding up well, very clever, I was dubious at first.
S.