The whole point of separating with VFR traffic in Class E is that they are not operating under a clearance and are not required to tell you if they alter their flightpath. So if you stop off a jet at FL170 due to traffic at FL175, and the FL175 decides to descend, what separation do you have?
NONE - you are just guessing and hoping.
Same with vectoring around VFR traffic - what if the VFR turns towards your traffic?
We rely on pilots of VFR aircraft talking up on the correct frequency and keeping a good lookout at all times. One day this will fall down, and we will wonder why we ever allowed it. If you are separating with the VFR traffic anyway, why not just call it class C?
Anyway, beating my head against a wall here. Awaiting the flack.
Cheers,
NFR.