First, notice that this question only makes sense for methods with reweighting steps that incorporate quotas. Now, onto some potential merits and drawbacks.
Once a candidate is elected, bullet votes for that candidate have expressed no more interest in the outcome. If they are not used now, they never will be, and more unused votes suggests a less representative outcome. Also, other ballots that give points to that candidate still have expressed interest in the outcome. Allowing them to keep their weight increases their chance of being used to elect another candidate. This again suggests a more representative outcome.
Potential problems include the fact that adding bullet votes for a candidate whose election was already certain may then result in another candidate being elected even though they never intended for their ballot to benefit that candidate. On the other hand, they expressed no preference for the candidate who lost their seat either, so maybe this isn’t an issue. The other issue is that following this rule to the letter violates IIA, since a vote approving the elected candidate plus an irrelevant one would not be a bullet vote. It’s likely impossible for a sequential method to obey this rule without violating IIA, since it can’t tell which candidates are irrelevant until the election is over. Optimal Monroe does both, though.