The ODbL is a license written for OpenStreetMap. Like a lot of other open licenses, it's not tried in court.
It's a 'sharealike' license: the main important provision is that combining ODbL data with other data requires the combined product to be licensed under the ODbL. In this way it's quite a bit more like the GPL and other old-fashioned open source licenses than MIT or BSD, and much less a license like Public Domain.
Why shouldn't you use ODbL for your project?
The sharealike provision is overly broad and has destructive consequences for potential large users: using derived works from an ODbL database, like geoocoding, routes, images, and so on, could make the entire new work under the ODbL. So, for instance, if you had a restaurant rating site, and you integrate geocoded locations into it from OSM, your entire database, including user data and reviews, may have to be 'opened'.
Don't misread this as simply being asked to keep open data open: this is a license that forces you to change the legal status of things that you simply combine data into. Your dataset can be 99.9% public domain and 0.1% ODbL: the license infects the same.
In comparison, a license like Public Domain is familiar, tried in court, and does not scare lawyers.
Does the sharealike clause encourage people to share who wouldn't otherwise?
No. Much like modern open source software, people participate in open processes because it's more efficient than 'forking' and trying to do all the work themselves. People don't participate in open source because they are legally forced to.
I want my project to be compatible with OSM: should I use ODbL?
No. Use Public Domain: you can happily import your data into OSM just the same.
I'm not in the US, so Public Domain isn't a thing.
That's why we have CC0: you can take your public domain to go.
Maybe you are right. Section 4.6 seems to say something different.
This is a tough license to interpret.