Hi Yog Sot,
I don't completely agree. Heatmaps are certainly a huge help in identifying unmapped roads and trails, but with due caution. For example, a heatmap doesn't necessarily indicate the presence of a highway: sometimes cyclists and runners cross fields or other areas, even private ones, without there actually being a "line of passage on the ground": these cases shouldn't be mapped.
Furthermore, a heatmap doesn't include the date of passages: you might have a red line, meaning many passages, in a spot where there was a road that was closed, or collapsed, a year ago.
But again, and this is also important, a heatmap doesn't tell you the type of highway: it could be a paved road, or a dirt road, or a track, or a path (or nothing at all!), etc., etc. So you run the risk of filling the map with incorrect highways, which remain there for a long time before someone can correct them... but in the meantime, they can cause problems for users who rely on OSM-based maps.
Ultimately, I agree with what Silversurfer83 writes and my advice is to only map heatmaps when the satellite photos are clear and recent or, obviously, when you know the place personally.