WARNING: Long nerdy dense post!
I was reading the article you posted on trehalose earlier boydster, and found a few more articles, and I don't think trehalose protects the yeast against the osmotic stress of the more hypertonic sugar solution in wort, or at least not as much as has been attributed to it here. Rather, it protects against thermal stresses, which is why it is in such high concentrations in dry yeast.
From the article:
Trehalose functions:
-™ Trehalose has the ability to stabilise protein
structure such that in the presence of this
disaccharide many enzymes exhibit resistance to
heat and desiccation.
-™ Trehalose protects cells from temperature
extremes by stabilising membrane structure
-™ Trehalose is a most effective agent for
preventing damage to membranes by its ability
to prevent phase transition events in lipid bilayers
-Trehalose synthesis requires metabolic energy,
whereas no ATP is generated in its dissimilation
(Boulton and Quain, 2001).
It goes on to say:
Trehalose accumulation in the yeast cells has
important consequences for different commercial
applications. The fact is already accepted that high
trehalose levels are important for producing active
dry yeast. In this case, trehalose offers protection
during drying process. It also offers cryoprotection
when yeast is used in frozen dough (Higashiyama,
2002).
My take on this is that it is high in dry yeast because that is necessary in the production of the dry yeast itself, and it is useful in rehydration.
I also found an article titled: "Osmotic Stress Signaling and Osmoadaptation in Yeasts"
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC120784/
It's a very dense read, which I obviously haven't read all of, but from what I've gathered, trehulose doesn't play much of a role in osmotic stress defenses, and the role it does play as an osmolyte is unclear. Glycerol, on the other hand, does primarily protect the yeast cell from osmotic stress, such as putting them into a hypertonic sugar solution.
The pathways for production of glycerol, trehalose, and glycogen all start with intermediates of glycolysis (Fig. ​(Fig.11).11). The three compounds have different physiological roles: glycerol serves as an osmolyte (7, 48, 63, 228), trehalose might be a more general stress protectant and assists chaperones in controlling protein denaturation and renaturation (173, 560, 561), and glycogen is a storage carbohydrate (173).
and later in the article:
It is, however, still unclear if trehalose can serve the role of an osmolyte in S. cerevisiae, as it does in bacteria (280, 580, 581). The role as a protectant may rather be specifically relevant for heat stress and extreme forms of osmotic stress, such as desiccation and freezing (236, 527, 561, 580). Hence, it appears that glycerol serves as an osmolyte for proliferating cells, while trehalose accumulation may rather aim at survival when proliferation has ceased.
and
The trehalose content of commercial baker's yeast preparations may be as high as 20% of the dry mass (622). This high trehalose content, which is achieved by growing cells under constantly low sugar concentrations in a fed-batch system, is believed to protect cells during drying and rehydration as well as serving as an important source for carbon dioxide production during the initial phases of dough leavening.
However, they go on to say
Numerous studies demonstrate a correlation between the cellular trehalose content and the ability to survive diverse stress conditions, ranging from especially heat shock and desiccation to starvation, oxidative stress, ethanol stress, cold stress, freezing, hydrostatic pressure, and osmotic stress (references 139, 158, 173, 429, 560, and 595 and references therein).
Which does mention trehulose as an osmolyte, but this is unclear, as shown here:
A possible role of trehalose in yeast osmoadaptation is supported by the Msn2p/Msn4p- and Hog1p-dependent stimulation of expression of the genes encoding all enzymes in trehalose metabolism (501), the reported sensitivity to high osmolarity of the tps2Δ mutant (61, 238), and the diminished survival after severe osmotic stress of mutants unable to produce trehalose (238). However, yeast cells do not actually accumulate trehalose to appreciable levels after an osmotic shock
Another article I found (which I was only able to access the abstract):
"Trehalose lowers membrane phase transitions in dry yeast cells."
Recent work has clearly demonstrated a direct correlation between the amount of trehalose present in the yeast Saccharomyces cerevisiae and its ability to tolerate dehydration, but has failed to elucidate the specific role played by trehalose. By using Fourier transform infrared spectroscopy we measured the transition temperature of phospholipids in both intact S. cerevisiae and isolated plasma membranes dried in the presence and absence of trehalose. Our results show that trehalose lowers the temperature of the dry gel to liquid crystal phase transition in yeast from around 60 degrees C to about 40 degrees C, thus allowing yeast rehydrated above 40 degrees C to avoid the damaging effects of passing through a phase transition. These results explain both the need for trehalose and the observation that yeast must be rehydrated with warm water if they are to remain viable. Only when trehalose is present is the dry transition within a physiologically tolerable range and only when the cells are rehydrated above 40 degrees C will they avoid passing through a phase transition.
This supports that trehulose is a protectant in thermal stress and dessication rather than osmotic stresses.
Okay, okay, if you're still with me, here's why I posted all this. It seems that after this recent discussion, it seems that the only reason left (aside from cost and ease, of course) to favor rehydrated dry yeast over a dry yeast starter is the high levels of trehulose to protect against the osmotic stress of a hypertonic wort solution. However, it doesn't appear that this is even a benefit of having a high trehulose content.