If a player has a 22% defense boost instead of 4%, will it really make a difference as far as what units you bring to the fight? Probably not.
I agree, overall the impact of knowing the bonus will be negligible on your decision making, which is why the ordeal of decoration hunting is even more pointless.
If you attack someone and they have a higher defensive boost than you're willing to try your luck against, you can always retreat with no losses.
The question is what is the benefit of the current situation i.e. does anyone actually scout for this, to avoid the chance that you'd need to retreat? Contrary to some of the suggestion here, I'd argue that most serious pvpers never will(at best they add/update this in a spreadsheet) and I believe that neither will the casual players who simply don't know about this and/or wont be bothered todo the decoration hunting each time for the negligible effect that it brings.
Note that OP was later augmented with a suggestion to add infiltrate/scout button to show that info. The button will be activated for a period of time for negligible cost.(10 gold for week? the standard theme, time is money) Which will:
a. Cause the casual player to feel like he takes more active role in the battles and provide him with more tactical info(always good), as well as avoiding the mind bogging pointless exercise in futility of decoration hunting each time.
b. Allow those who want to scout and the "pro"s who don't need this info, to continue todo as they did. However, since they don't waste the 10 gold on this, it might make their effort feel more rewarding.