I insist on them on large new build housing sites in conjunction with tanking.
If the shower tray/bath are of good quality and square vertically i.e: the upstand and top are 90 degrees rather than sloped from a cross section point of view then the Classi seal installed correctly is no obstruction at all, it's only about 1-1.5mm thick.
If when installing it the plumbers silicone it to the wall and have it nipped around the corner of the tray properly and not stretched it, then held the corner in place with a piece of wood scrwed to the wall until the silicone sets, when we come to tank it it's not in the way what so ever, it's a clean 90 degree and vertical internal angle.
I do agree that there are other options out there but I haven't found any that are more cost effective when tanking large volumes of new houses with a restrictive cost to build.
