Ultimately, who can do it the best? The art of fugi God takes many years to achieve. I can silicon better than most people, however if the tiler does a shoddy job i get ****ed as i could have done it better.
My father had two new build houses built a year ago. The chippy/builder did the silicon its shocking. I would of much rather done it myself. The specialist caulking firms that do big jobs and sites aren't particulary good at it either all there interested in is uniformity and meterage. So as long as it all look the same they dont care.
I have a Japanese silicon gun in mettalic dark red. Did a bathroom in january had 6 shades of Grey silicon.