Based off of a comment here

At one point, there was some performance advantage to having mapTo, concatMapTo, switchMapTo, et al. Because they introduced a way to map to the same value over and over again, without introducing a closure.

Now:

1. Closures are much faster.
2. Therefor the whole library is written in such a way that is uses closures.

Considering that mergeMapTo(x) and mergeMap(() => x) do pretty much the same thing, only one is hiding the closure, I thikn we should deprecate the MapTo variants to help clean up the core API.