An HTTP request has to be sent on each tracker for each video to spy. If we want to spy all PeerTube's videos, we have to send as many requests as there are videos (so potentially a lot)
It can also be displayed on external websites to promote your instance, such as <x id="START_LINK" ctype="x-a" equiv-text="<a target="_blank"href="https://joinpeertube.org/instances">"/>JoinPeerTube.org<x id="CLOSE_LINK" ctype="x-a" equiv-text="</a>"/>.
The worst-case scenario of an average person spying on their friends is quite unlikely. There are much more effective ways to get that kind of information.