My comment regarding the 'cosmetic' issue was only made in direct response to the chorus of 'ugly, etc.' type comments ... obviously based upon that entertaining image first posted.
As far as the mechanical aspects of introducing a manual safety? I'd offer that it sort of depends on what's involved.
Folks could easily feel the same way about the grip safety, you know.
Anyway, these things are probably going to be increasingly found on pistols in the coming years as LE/military requests may start to include them in specifications.
The M&P thumb safety on a couple of their MP45 models was deliberately designed with a military specification in mind, and its function is pretty simple. The safety lever function serves to block rearward movement of the trigger bar when in the UP position. Not all that complicated.
Dunno what Springfield requested done to the XD design to incorporate their new variation.
I'd suppose that Springfield felt this option might have some commercial interest at some point or else they wouldn't have requested that the design be revised to incorporate it.
FWIW, I only opted to buy my MP45 with the thumb safeties because I have many years of using a similar design (and similarly located, more or less) on my 1911-style pistols, so I already have some trained, ingrained patterned motor skill responses to use, and I sort of wanted it designed as it had been in anticipation of military trials.