Do you think the Steam Workshop will be ready to download really complex mods like Total Conversion mods such as FCOM from Oblivion or FWE from Fallout 3?
I don't think the workshop will ever support loose files and that is going to limit what is possible greatly.
I doubt they want to either as that would dash any illusion that using SW is easy or convenient because to do this leads right into needing to track what loose files conflict and replacing as needed which then leads into what makes BAIN and FOMM really great programs.
So even if they do incorporate these ideas it will be a good while.
Also directory thrashing - I bet that has never been addressed. SW users are going to be up against that faster than users of BAIN or NMM.
and as for Nehrim - even the Nehrim developers didn't get installing it right and it will mess up your oblivion install.