There are a variety of reasons why it's good to be able to "combine layers" for Support and for regular Infill. For example 0.1mm layers print very slowly and make terrible flaky support and infill structure. I would have no problem printing those at 3x so 0.3mm height.
Well the same principles apply to Solid Infill a.k.a. top and bottom layers. It would give a lot of flexibility and help solve a number of problems if there was a dialogue to "combine layers for solid infill". (In my 0.1mm prints I'd be doing solid infill at 0.3mm right away!)