Occasionally someone reports that they've installed tyre X on bike Y and have "no clearance issues". Others who have bike Y then wonder whether this might be a good idea.
The desire for this perceived upgrade seems to be coming from a culture of bigger-is-better. Even a racer might prefer 700x28, and wider MTB tyres provide more traction.
In my mind one should more carefully follow manufacturer recommendations. Even if 700cx28 fit when 700x25 were the supposed maximum, if 29"x2.6" fit in a place meant for 2.4", or if 26"x4.6" fit in a place meant for 26"x4.0", this extra clearance is there by design to leave space for sand, pebbles, and mud or snow respectively.
Is this about right? What's to stop someone from installing the largest tyre that'll fit?