What is risk?
According to dictionary, risk is defined as
a possibility of harm or damage against which something is insured.
In the case of robocode, if your goal is percentage score, risk is the possibility of harm against your final percentage score. For example, firing lower power bullets in melee reduces your potential score gain comparing to those firing higher power bullets (not considering hit rate here), thus increases risk, while firing higher power bullet when you are low on energy and having good chance to dodge bullets also increases risk, as you expect to earn more score from survival than doing more bullet damage in this case.
Experiences on energy management in both melee and 1v1 proves the above rationale. Now the question is:
is risk in minimum risk movement properly defined?
Having looked at a few open source melee bots, the risk is mostly a combination of some estimation of bullet damage from enemy & distance. However, being afraid of taking damage may not be the best choice to earn robocode score. For example, an extreme case, against a sitting duck, and when only you and the sitting duck is left, and you have more energy than the sitting duck, ramming it gives you more robocode score than staying far away while firing at it.
It makes a lot of sense, especially if you're looking for a fitness function for a ML algorithm. Optimize for points gained and minimal points conceded to the opponent. Can't get much better than what the rumble uses!
I think there is a fine line between successfully aggressive and too aggressive. You want to be aggressive enough to deal maximum damage, but passive enough that you survive long enough to pick up the survival bonus (and stay alive to deal more damage). Since the level of aggression varies depending on the bots you face (very aggressive against sample bots, passive against the likes of Neuromancer, ScalarR, and Diamond), its difficult to decide how aggressive to be, and since you lose much more score in being too aggressive rather than too passive, many bots choose to go as passive as possible.
That makes sense, even if being aggressive sometimes gives you a little benefit, whenever you made a decision mistake because of unavoidable classification failures, the loss is great. So the line should be set far from danger, and even if one go beyond the line a little bit, it’s still fairly safe.
Although sometimes go beyond the line a little gives a little gain, still being fearful is the price we pay for uncertainty, and actually helps us being safe and successful after all.
In melee, being aggressive in the start of the game, gives you the benefit of 'cleaning your neighbourhood' and therefor can give you a relatively safe area when you survive till the last 4 or 5. Plus the bonus of doing a lot of damage. Then the conservative strategies kick in, like trying to never be the closest to anyone else and saving your energy.