Any change implies different behaviour, And if there is code depending on the previous behaviour, that code needs to adapt. It's always that way, uncomfortable to some point, and things change nevertheless. It is otherwise impossible to progress. So that is for me not an argument against change.
On to the topic: I'm here doing something few people do these days: listening. So far three people said things. While each person thinks her opinion is the good one, each opinion is only one in a universe of opinions. At my end, I am in charge of the simplifier, and it is my opinion that it is good to listen to others' opinions, think, then decide.
Int and %int currently behave differently under simplify. That is incorrect and, of course, we can continue incorrectly, but generally speaking, I prefer to fix these things. In this moment, as noted by @Preben Alsholm, simplify(%int(x, x)) returns x^2/2, unlike simplify(Int(x, x)) that returns Int(x, x). Whether that is convenient for you because you use Int instead of %int doesn't change the matter. Int and %int are two-supposed-to-be-the-same thing, the inert form of int.
The proposed change under consideration goes more into a) making simplify@%int behave like simpify@Int, so do not value things, b) in some cases we may want some equivalences detected - I mentioned some examples above, asked for "Opinions?"; and c) for Functions, (not int, diff, sum, product or limit), for now we may want to keep the current behaviour, where things like %exp(0) become exp(0) when exp(0) is something simpler.
To the side, your comment about "do not remove usefulness" sounds to me a bit out of place. Not only for what implies for me (the guy who removes usefulness? I feel more like the guy who perhaps thinks differently than you) but also because it could be read as "you think that whoever thinks differently than you is indisputably wrong." @acer , that is not useful. Let's just collect some opinions without disqualifying anyone's.
Edgardo S. Cheb-Terrab
Physics, Differential Equations and Mathematical Functions, Maplesoft