Why? If you’re rolling the first boon, decon 2 is more cost effective. At least according to my calculations 
Let’s assume rolling the first boon takes you
- 2 gums, most often special or effect
- 1.2x boon point generator, like pbc 3 or corrupted 2
The more expensive ones are
- 6.2c each effect gum
- 54c each corrupted 2
Decon resins have a price difference of 20c
With decon 2, on average you’ll return 6.2c + 27c = 33.2c worth of items
With decon 3 the numbers are 9.3c + 40.5c = 49.8c, difference is 16.6c, less than the value difference.
If you’re simply farming the stuff yourself and don’t care about the coin values, I’d argue shimmering and shadow orbs are some of the harder items to collect when comparing to pulsating orbs required for decon 2. I’ve been happy with decon 2 in my deck, but that’s just my 2c. Curious what other people feel is better 
At least in my forges I rarely have to decon after I’ve got the boon setup I wanted, maybe there’s a place for it if you decon often at a different point.
(Reasonable prices taken from Boundless Trade Network - Now Automated and calculated with Shopkeeper's Price Calculator / Recipe Browser if somebody wants I can provide the numbers I used and more logic behind some of the numbers above)