So even if you put a link there you need someone to be aware that such a link exists, to believe there might be something useful at the end of that link (which you point out, there rarely is), then wants to go to/turn on their computer, fire up iTunes, attempt to & succeed at finding your app, click the link, possibly register for your bug tracker (doesn't seem too uncommon), possibly check their email to confirm their registration and then finally file a bug report.
Anybody can feel free to contact me about this via my profile here, I'd truly be interested in making this flow easier and I know how to write this stuff.
Personally, a "report a problem" link would be a lot more inviting, especially if it let you send them a message from the app store, and include the last crash report (if it was the last one).
Oh wait -- they do! It's just a shame that most developers don't put anything really useful in there.:(