Jürgen,
I abandoned using certificates for SAP sites a couple of years ago. When they work, they work well. However, they always just made troubleshooting SCN and Marketplace access problems a bit more complicated than they had to be. I am not bothered by having to access SCN and other SAP sites by entering logon credentials. You may remember years ago (before SSO certificates) when this was the ONLY way to access the non-public SAP sites.
I also have to have two browsers running when I access SCN, and sometimes I actually have to revert to a third (Firefox) when the other two (IE and Chrome) don't work. I will admit that find this problem not only with SAP sites, but also other non-SAP sites.
Is this state of the art? Probably not. It could definitely be better.
Will SAP make it better? We'll see. If you really look hard at SAP's web presence, and look at all of their software as well, it is easy to reach the conclusion that SAP doesn't really care all that much about usability.
Best Regards,
DB49