Enchytraeids - Pot Worms

Its been too windy to do much at the plot today, so instead I was repotting things in the GH.
When I came to empty out one particular pot I noticed that it was infested with 100s of little white wriggly things - maybe 1cm long and a few mm wide. I hadn't come across these things before - and as the cucumber that had been growing in that pot had just committed harakiri/seppuku I took the safe option and emptied the compost out into the green bin.
On looking up what the critters were I discover that they are pot worms that are actually harmless/beneficial - the cucumber must have had its own reasons , "bereft of life, it rests in peace"
I was wondering how common they are ?- first time that I've ever seen them as far as I can recall.
http://www.happydranch.com/articles/images/clip_image001_012.jpg

Cheers,
Balders