Bug heat
Launchpad helps you to appraise a bug by giving you a calculated measure — called bug heat — of its likely significance. You can see bug heat in bug listings, and also on individual bug pages, as a number next to a flame icon.
Here's how Launchpad calculates the bug heat score:
Private Adds 150 points
Security issue Adds 250 points
Since users should be encouraged to submit and share information of bugs, why does keeping a bug private get additional bug heat score? If it is related to a security issue, this might be OK. But in fact at most time when apport returns "Your bug is a duplicate of an existing bug", this bug is private and I cannot submit any useful information.
Is there any other reasons for this rule?
You’re correct in that the assumption is that private bugs usually are so because they contain private/sensitive information (also, undisclosed software vulnerabilities waiting to be fixed), so in theory developers should act quicker on those. The increased bug heat should indicate a greater sense of urgency. But all of these are just assumptions made by the Launchpad designers. In reality, the score bump is just an arbitrary number, and IMHO the “bug heat” isn’t as effective as, for instance, the Importance field, to indicate what bugs you (as a developer) should focus on.