Some users do not allow their profile to be viewed by everyone (privacy) and reporting such users for their avatar, signature or other content not post related has to be forcefully done via reporting a message, which at times is confusing and a small extension to the whole method.
Adding a report button to the member card for that particular member will still allow people who cannot view said members profile to report them (report the account and not any content.)
With profile privacy playing a role in members not being able to view profiles, a report button on the member card is only a logical thought and implementation (imo) to ensure that reporting the member works regardless of profile privacy restrictions. Otherwise, to a member who can't see someone's profile (whom they wish to report) it's just as good as the feature not being there (and also adds extra hurdles to the whole report process, having to find a content from that member to report + explain).
Example - Facebook and Twitter. Both allow reporting a person directly (non-content reports), and even if the member profile is private you are still allowed to report it by the hover-over member card/info or pop-up member info/card.
And I also presume the other links (such as view profile, follow/unfollow, ignore etc.) were placed in the member card so that members unable to view profile due to privacy (especially with ignore) could use the button there, and it makes logical sense to also have the links placed there, so why not report? If directly reporting a member has been implemented as a feature, it should also allow reports from the member card for reasons stated above.
It's not completely important or really urgent to have the feature, but just something I feel should have been there from the time the actual feature of reporting members directly was implemented.