They do not believe the strength of silicone glue. By just using silicone glue without U channel they think the glass will come off. In their minds they only see that only if screws and/or bolts locked into concrete will it hold. With U channel they will drill holes into the floor  and screw the U channel into the floor and wall and then the channel is filled with silicone and glass is place into the channel.

Frameless is elegant and classy just like a one big piece of frameless mirror, compare to a framed mirror with stainless steel frames on two sides and two other side no frame, not nice.

As a diyer before doing the job for the first time we usually do testing and experiment and sometime even prototype. I tested the silicone glue on the left over tiles and acrylic which I would let the shower screen glass sitting on top of it. Here is the result of the test in the pix. I could not pry open the silicone that glued between the acrylic rod and the tile. To force it I would break the acrylic rod. The only way was bending the acrylic rod and using a blade cutting the silicone glue. Unlike acrylic rod, as for the tiles that cannot bend I could not even insert the blade in between them. See pic, hanging them on the tree.
[attachmentid=3862039]

Also, I tested the acid based silicone. It just couldnot stick on to the acrylic. But that is a different story for the neutral based silicone, it stuck as per the experiment.

Acid based that cant glue to acrylic plastic
[attachmentid=3862074]

100% RTV Neutral Silicone Glue that works well with acrylic
[attachmentid=3862079]

Not only that. I also compared and tested the imported GE silicone sealer as in the picture above with this locally made X'traseal. The locally made proved to be better The GE USA made costs RM24 for a small tube. The xtra seal only RM7 a tube.