In this months YD is an article on this ( p.96)  and it states that pyometra is avoided as the ovaries are necessary to contract it .
I personally would rather get rid of the womb in a bitch of mine as  i wouldn't be breeding , and if it's not there it can't cause a problem .
I appreciate that there is less scarring etc , but my bitch didn't need a collar after her op as she didn't bother the stitches . Also , she made less of a fuss than my tom-cat when he had his b@lls off !!!!
Opinions please ........

There is still the rare situation of stump pyometra, even where a standard full ovario-hysterectomy has been performed. The problem is that, because a bitch is known to be spayed, this condition is very rarely considered, so can get quite serious.

I think they have been doing this for a while so presume they have gathered a fair bit of evidence to support it.  I guess they are saying pyometra is unlikely if the uterine horns remain in a tiny state without developing a lining but like you it does make me feel a little uncomfortable as avoiding pyometra is about the main reason I have had mine spayed in the past.

When my bitch got pyometra she was spayed and glued back together, there were no stitches for her to nibble on and she recovered faster than any other bitch I have had spayed. I assume that to do the op removing only the ovaries the bitch should not have had a first season? If she had had a season would she not still come into season? Anyone know?

No she would not have seasons because the ovaries dictate this, hence if a small amount of ovarian tissue is left following a spay they continue to cycle into season and attract males.