We dadoe the bottoms for strength. By cutting a 9/32 inch groove into the sidewalls, inserting and gluing the bottoms into this groove, it ties the whole kitchen drawer together. Also, on kitchen rollouts 25" and larger, it allows an area underneath of the bottom to add a strengthening  board to reduce sagging. We have seen kitchen pull outs that use a 1/4" board that is surface mounted to the bottom and over the years, this bottom will sag.

Also, if you surface mount the bottom, you have to attach the glides with screws in the bottom rather than from the side. When this screw is inserted into the bottom, it splits the ply's apart on the sidewalls unless you drill pilot holes (which no one does). If you see a kitchen shelf advertised as being 2 5/8" tall this is a clue that the shelf sidewall is actually 2 3/8" tall and they are counting the 1/4" bottom that they are surface mounting to the sidewalls. These shelves are usually also butt jointed. This is the worst way to build a pull out shelf that will get daily use. These shelves are usually only warranted for a couple years.

What about mounting a 1/2" bottom and forgetting about the dadoe joint?

The 1/4" mdf bottom that we use will hold more weight than the drawer slides are rated and being able to dadoe them makes the total drawer stronger. Why would anyone surface mount the bottom to a drawer? If you use 1/2" particle board, you just added additional unnecassry weight to the drawer slides that have a weight load rating, meaning you just took away the total weight you can put in the drawer. Also, consider that when the bottom is surface mounted and 3/4 extension drawer slides are used, the drawer slide will hold the outside edges of the bottom the the drawer sides. But what about the front and back? Adding and subtracting weight from a drawer that has a bottom surface mounted is the same as bouncing on a bed, eventually (and not that far away) the springs loosen up and you have a sagging mattress ( or shelf bottom in this case