When downloading from "reputable torrent tracker XYZ" you can trust the quality of the torrent, that it is virus free, etc ... It is also usually make searching for particular torrent easier (less like searching for a nail in a hay stack) and you avoid spreading the seeding potential to hundreds of similar torrents.
As a extreme example, BakaBT (a private torrent for anime/manga related torrent) has a strict "no duplicate torrent" policy. This means that if you are searching for the OST of a specific show, you will have usually only one result and it's the most up-to-date, highest quality version. Since it is the only option, everyone seed this one. It really diminish the issue of abandoned torrents. To "replace" and existing torrent, you have to provide a strictly better version.
A decentralized torrent search engine could not do that. The real value of torrent tracker are the community.
That is also why decentralized software like eMule/eDonkey lost a lot of popularity to torrent tracker: Lots of duplication, very dodgy download, no curation, virus, ....