How hard would it have been for Honeywell to slap out a craptacular prototype product to be "in the market" and then sue Nest? Not hard, if they cared to do it.
How hard would it be for Nest (or another small company), if they had the original patent, to bring a product to the market in order to qualify to sue Honeywell? Maybe not impossible, but surely harder than the reverse.
If I grant that patents should exist, or at least that they DO exist, it seems reasonable that reducing an invention to practice (but not necessarily to marketability) is enough to go seek a patent, such that you could then seek funding or partnership opportunities at far more favorable terms than if all you had was a secret invention that you couldn't patent until you were able to bring it to market with money you didn't have and couldn't get.
1-excepting perhaps in drug discovery cases, where I am less certain of what I think