This has come up a couple times before over the years. I've never been a fan of "rep" systems. While it's a nice idea in theory, the implementations never seem to truly lend themselves to reality unless a site has VERY high traffic levels. We're only a small-medium sized community, so the "truth" of a rep rating tends to be skewed. It becomes more a popularity thing. But also, they exclude new members who may be very knowledgeable, and can be frustrating for those who actually do know what they're talking about, but haven't "built up" a solid rep yet.
It would be a good idea... but just hard to get it to be truly meaningful. Always a possibility, though... I've not seen many that I actually like that plugin to phpBB, so I'd likely have to create one myself. Also, most out there plugin to phpBB by modifying the base code (called "mods" because they modify the code), which I am not a fan of. Each mod added to the system makes updating standard updates all that much more complicated and risks breaking the site. If I were to implement it myself, (for those into the techie stuff) it would be to modify the templates for the site, and use the support for having code in the templates. It's generally frowned upon for various reasons, but in my opinion, those reasons are minimal and templates are already considered "distinct" for a site. Standard updates don't break custom display templates and vice-versa.