Yer.. I use drainoffs as well. but only on bottom floor rads and when they are against a wall. Underneath a window I want to keep the rad -exactly- central. In that case (the majority of the rad replacements I do) I use an extension on both sides.
I've been experimenting with these connectors for the last 2 years and theres a few tips. Firstly, the basic constuction of these is an 'oyster' style connector. a 15mm tube inside an 'o-ring' seal. So you can extend these if you want by replacing the exisiting tube with any length of 15mm...