switches for the main line

I use a pair of 7.5 for a main line crossover in an area of the layout that can't be seen (monitored). I use the number 5s in conjunction with the use of double slip switches which are of the same switch angle. You may have to take steps to add additional roller pickups for short engines or they won't make it through these switches due to a wide separation of powered center rail. Also, long engines that have forward facing rollers can drop into the large gaps on these switches and STOP the engine dead in its tracks, so to speak.

Look at the Atlas O website, in the track section.  They provide details about track center distances when a crossover is made using various sizes of switches.

You are right, though, that those switches don't "replace" any standard curve size, so best idea is to only use them in a long straight section of parallel mainlines.

Chuck