Phil V wrote in post #17876768
Because, like I said, the light is in the wrong place, behind a lens and within a reflector.
Some people do it, and are happy with the results, but IMHO it's not the same light as a strobe where the flash tube is sat in the right place.
It's not going to be quite the same, but how much so depends also on the specific beauty dish. There are different designs for the center disc (convex, concave or a flat) and different dish shapes. The concave mounting (bump side towards the flash) works best with a speedlite in my testing, giving noticeably more output. I was skeptical at first, putting a single 600ex-rt into a BD, because I thought the rectangular shape of the flash head was going to make the dish light up in a not evenly-round way. But to my eyes anyways, and with my ordinary shaped white-interior dish, this turned out to be a hypothetical concern, and not something I can actually see in catchlights.
If you have an undulating dish like a Mola or Kacey copy, particularly the silver ones, then you will have a problem, since these really do need the symmetry of a circular flash tube to make their perfect concentric rings. But with a more ordinary white bowl-shaped dish, a speedlite set to a wide angle fills it pretty well, enough IMO to get a beauty dish look from it. Is a strobe better? Sure, but I don't think the speedlite result is going to be so bad it's worth discouraging someone from trying it out, and if it isn't good enough, well, you've got a Bowens mount dish you can put a strobe on!