Spit! Yuck! Rubbish! Balderdash! And other such words...
Convertibles were the cheap option, when slates all cost well over £1,000, but you could pick up a convertible for closer to cheap laptop prices. I know, because I had one. More importantly, I had one, but no-one else did. I've seen a few slates in the real world, but the only person I've ever seen use a convertible, is the guy I sold mine to.
There's nothing wrong with them as laptops. And from having one, I'd say that touch on a laptop screen is worth almost nothing extra. I did use it, but very, very rarely. Although Windows 8 may change that, with those chunky Metro tiles. I doubt that personally, but anything is possible.
As tablets though, they're way too heavy. I find my iPad a bit on the porky side after a couple of hours of reading an ebook. And that doesn't have a huge keyboard attached. I didn't read ebooks on my old Tablet PC, otherwise my arms would probably have fallen off by now.
On the other hand, detachables are great. If you can successfully resolve (or fudge well enough) the required design compromises, then you've got the best of both worlds.