I get the charm and it's quite touristy when you arrive by ferry, walk along the cleanish port and enjoy the sunshine and breeze and hey, Freddie Mercury museum!! However, venture deeper and it is dirty, hazardous, on the brink of collapsing (word is the site will be placed on the in-danger list soon). I think the idea of the entire old town being inscribed doesn't quite cut if anymore. Sure there are random houses, doors and windows from the classic ages but they have either been destroyed or replaced with only a few spots being highlights. In its defense it rained the night before and even in the morning I visited so the roads were dirty and zero fun to walk around. There are also motorbikes and some roads allow cars meaning the car-free zone is mainly where the shopping alleys are. It doesn't help that the north end of town is the ever-busy fish market and the east side a bazaar (not the touristy kind, more of outdoor market for the locals).
The history of the old town is important but neglected care and financial commitment, plus from what I read the inscription was more or less a favor or acting in good faith that it will be handled by the semi-autonomous region. Fail.
As it's an easy visit and any visit to Zanzibar (especially by ferry) makes it an obvious itinerary inclusion, and even a day trip from Dar Es Salaam is doable. In hindsight I'm glad I only spent a morning on it before heading off to the beaches.