New alcove tub - ledger or no ledger?

Was looking at the rough-in and installation instructions for a Kohler Devonshire acrylic tub with tile flange and apron. It will sit in a 3 wall alcove, 59" x 32".

Question is whether or not to use a legder board to install the tub. I have read several DIY books which say to use the ledger. The Kohler instructions say nothing of this. My thought is that if the tub rests on the subfloor, supported by mortar and construction adhesive, and is nailed to the studs through the tile flange, this is the acceptable installation. I can't see what a ledger board is doing in this case. The tub is sitting on the subfloor, not the ledger, so why have it?

The ledger board is there in case a lot of weight is applied to the rim so that it does not deflect, allowing it to separate from the wall. Most tubs don't recommend drilling through the tiling flange and screwing the thing to the wall. Many of them don't have a flange sufficiently tall to allow this, either. Since, apparantly, the Kohler builds their flange thick and tall enough to handle this addition stress, then it appears you won't have problems by eliminating a ledger.

Kohler manual

Good explanation. I looked at the installation pdf for the K-1184,
http://www.us.kohler.com/onlinecatalog/pdf/1014139_2.pdf and true enough they do tell you to drill holes in the tile flange, which doesn't look exceptionally big. No mention of nails though.

Wow! I sure hope the Tech Suppport people don't discourage anyone from putting in a ledger board if anyone were to ask. Ledger boards are strong because they distribute stresses along a flat top, whereas drilled holes are weak since they concentrate stresses onto a single edge of the hole. Vibrations (e.g. a motor in the basement) will eventually cause the plastic (polymer, fiberglass shell, acrylic coating, etc) to fail at that point, years from now. Any sized straightedge board supporting under the tile flange will work.

The installation manual also mentions using mortar to support the middle of the tub while ensuring that all the support legs are firmly standing on the ground, and to fill the tub with water etc. Good !! At least one manufacturer does put it in writing. Now we can refer people to Kohler manuals for that.

But not for ledger boards.