I think its a good idea.
It eliminates (any excuse for) a lot of the friction and discontent regarding track lay-out, track condition, and riders.
It would also mean a much less packed schedule for organsiers to try to fit into a weekend.
The Evo/Pre-85 event could even run pre-90 as a demo class if they wanted.
The main bad point that I can see, is that someone who wanted to chase championships in across both eras would need to attend two different events (meaning more time off work, etc).
On the flip side, it would mean that they could concentrate on a particular era all weekend without having to re-adjust to wildly different bikes during the event.
Edit: I'd include the pre-78 bikes with the pre-75 and older bikes, rather than with the Evo+ bikes. This makes sense based on the number of riders in the classes (at events I've been to), and the bikes (remembering that most pre-78 bikes are closer to 7/4" of travel than 12/12" that's typical of the Evo bikes).