I agree that IDY is more convenient to use, and less prone to rehydration problems because of incorrect water temperature, but there are people who use ADY because of its higher concentration of dead yeast cells, as evidenced by this post by forum member November at: http://www.pizzamaking.com/forum/index.php/topic,3985.msg34030.html#msg34030
(Reply 18). There was a time where King Arthur sold dead yeast cells as a dough conditioner but I believe that that is no longer the case. There are commercial sources of dead yeast, however. In November's case, he uses ADY primarily because the live yeast can cannibalize the dead yeast for nutrients not found in the surrounding environment. With ADY, there's more dead yeast to cannibalize. In fact, about 70% of the cells in ADY are kaput.
There are also applications where ADY may be favored over IDY for certain dough applications, as noted in the following excerpt from theartisan.net yeast treatise (see especially the portions I have highlighted):Active Dry Yeast
Active dry yeast begins as compressed yeast but the press cake is extruded through perforated plates or screens in the form of thin spaghetti-like strands. These strands are cut into elongated pellets as they enter a tunnel dryer and pass through a series of drying chambers maintained at different temperature levels. (Drying may also be carried out in rotating drums or in the fluid bed system.) The pellets are then ground into small granules or beads. The combination of strain chosen for active dry yeast, the growth conditions, and the drying method tend to favor stability over activity. This means that active dry yeast has lower activity or gassing power than compressed yeast in lean dough. Active dry yeast has lower activity or gassing power than compressed yeast and higher activity or gassing power than instant active dry yeast in sweet dough.
Instant Active Dry Yeast
The manufacturing process for instant active dry yeast is similar to that for active dry yeast with a few exceptions. Ascorbic acid may be added as a dough conditioner to help strengthen the dough. Prior to being extruded, the press cake may be plasticized with sorbitan monostearate (an emulsifier) as an aid to yeast rehydration in the dough. The yeast mass is extruded through smaller perforated plates or screens than those used for active dry yeast, cut into small oblong, thread like particles and dried in a fluid bed dryer. The combination of strain chosen for instant yeast, the growth conditions, the drying method, and the addition of emulsifiers tend to place instant yeast intermediate between compressed and active dry yeast relative to activity or gassing power in lean dough. Instant active dry yeast has lower gassing power than compressed or active dry yeast in sweet dough.
Some people also use non-rehydrated ADY in order to slow down the fermentation rate, or its onset, which can have the effect under the proper conditions of increasing the useful life of a dough. In this case, the live ADY cells are entombed by the dead cells and it takes longer for the moisture in the flour to penetrate the dead cells to get to the live cells to rehydrate them.