[p_l]

One more thing to consider is memory bandwidth, which can be limiting factor especially on mobile devices.

A non-transparent draw over another draw allows in best case to cull all overlapping drawing operations, in worst case means you only have to use as much bandwidth as the individual draws.

With transparency, especially if you can't somehow combine the operations together (from my understanding, very hard problem), it means you also need to read back the entire region you're overlapping with transparency - so every transparent draw involves at least twice the final framebuffer size bitmap going-over memory bus.

Now consider that many mobile devices had not enough memory bandwidth to do a full redraw (i.e. blit the full framebuffer image twice) of the screen in time to maintain 60fps and it becomes a considerable problem.