Launchpad lets you subscribe someone else to a bug report. Occasionally I find a bug closely related to one that Person X is already subscribed to. I therefore think X is likely to be interested in following the new bug, so it makes sense to notify him in this way. On the other hand, spamming X with extra e-mail can be considered poor etiquette.
Is subscribing someone else acceptable? (And if not, then why does Launchpad permit it?) What about subscribing a team to a relevant bug?