We had three brand-new toilets that leaked slightly, apparently through the flapper. Even if it wasn't a significant amount of water, or caused a noticeable increase in the water bill, it caused an unsightly red mold to grow. Even though the toilets are cleaned weekly, the mold can be seen as vertical stripes in the bowl wherever the leak runs down the sides of the bowl.
Anyway, Fluidmaster now makes a new fill valve that refuses to refill if there is a slow leak. As soon as the handle is jiggled, however, it will refill. So a tenant can use it just fine with minimal inconvenience, but the normally wasted water is saved.
This may increase your callbacks in new single family construction, but it's the perfect solution for buildings with maintenance staff.