Multires : Apply base should be done in realtime and fast.

Apply base should work in realtime whenever you make changes to high subdivision levels, the changes should affect the base level when you go down.

I know there is sculpt base mesh option but it's slow as heck for sculpting.

In Zbrush, Nomad, 3d Coat and so on all have their subdivision levels working in the background and fast for high polycounts except for Blender where you have to push a button each time.

I know comparing with the other softwares is not a good idea , but just to point out that Blender is the only one who is doing it in a slower way and making it hard for the multires workflow.