I think we've reached the same state as it was in CS6.
Multiframe rendering for final render - GOOD
Multiframe for previewing during work - BAD.
It seems that every time AE starts multiframe rendering it starts with launching background processes that are going to do the rendering, which is understandable, except AE does absolutely no rendering untill these bg processes start. And since it takes around 15seconds!!! to start the processes, we 're stuck waiting 15seconds every time we hit the spacebar - how do you expect people to work like that?
Seriously, on many comps, 15 seconds is enough to render a good couple of frames and decide, that fixes need to be made. I remember MFR in CS6 - I often turned it off during work and turned it back on for final rendering for this very reason.
It's obvious to me that the MFR should only be an addition to normal single process rendering - I need to see rendering happening whenever I press space, if that means that the bg processes will be starting for 30 seconds instead of 15 - fine, I'm cool with that.
You simply cannot expect people to wait 15seconds every time they press spacebar - I do previews like that sometimes even 10 times a minute, when I'm tweaking a value in an animation.
So now what - same work will take me 4 minutes, or am I supposed to again turn MFR on and off all the time? - If so make a button for it near the home icon on the toolbar.
I'm not a programmer but I think adding a sub-option to only user MFR for Rendering in the Queue/AME should not be a terrible hard thing to implement. I get that the goal would be to have the MFR spin up in an instant but maybe, just for usability sake, until everything is in order... maybe give us more choices or at least easy access via a checkbox in the Preview window or something similar.