@marcusprit: Main reason, for me, is that both Miele and Riccar/Simplicity vacs seem to move quite a bit more air than their Sebo equivalents. Also, the Tacony PN and the EBK360 used on some Mieles (SEB228, IIRC) have *fantastic* agitation, better IMHO than the Sebo ET1. Mind you, the ET1 isn't bad by any stretch - this isn't good vs bad, it's outstanding vs excellent, I think.
I can't say I've done a truly scientific comparison - I don't really have the equipment to do so - but my Simplicity Verve pulls more dirt, especially heavy grit, behind my Kirby Heritage II than when I run them in the other order. Last vacuum wins and all that, but the Simplicity seems to budge the heavy stuff better than the Kirby, and the general consensus is that Kirby does a pretty darn good job.
When I try the same comparison between the Verve and a Filter Queen 95x with an EBK360, it doesn't seem to much matter which one goes first, the other gets about the same amount and kind of dirt. I'd expect an EBK360 on a Miele to be similar, though I don't have a Miele to test with. A friend of mine does, though (the Kona, IIRC), and she loves it.