Recently, Bonus XP seems to have fallen out of public favor. It obviously has no place in, say, power min-max metagaming campaigns where surmounting nigh-impossible challenges is the main point; yet, even in more roleplaying-heavy and/or non-serious campaigns I frequently find it advised against.
On one hand, I understand it's primarily used as "training wheels" to encourage new players to be creative. Yet, at the same time, I can't see any good reason to not use it, especially since particularly notable moments that are inevitably remembered for years to come seem deserving of some recognition (and I sure as hell don't feel like designing a Renown mechanic... er, actually, that does sound cool. But, I digress). And, besides, I rarely give it in large enough quantities to unbalance the game (the sort that are commemorated in large illustrations for custom screens and game mats among my group).
So, for what objective reasons based on things such as game balance, player psychology, and the likes would it be a bad idea, if any? I'm not interested in "well I stopped using it at my table in '98 because I didn't feel like balancing it". I am looking for "I noticed this incentive made my player do x undesirable behavior" or "Small rewards like these, in any quantity, often break system y". An example would be "I noticed my players intentionally put themselves in harm's way to get a climactic escape and earn the incentive instead of just playing pragmatically". Tl;dr: Objective, tangible cons to using this system, not subjective opinions on why it's bad.