There is some confusion in this thread.
A single data item is not licensable. The fact that Berlin is the capital of Germany, or that the population of the Seychelles is 84,000 is not licensable. Let us hope that this will never change.
Copyright covers a specific expression, e.g. a concrete text, image, etc. For example, the Britannica article on Germany or the Wikipedia article on the Seychelles might be copyrighted, and thus can be licensed with a license like CC-BY-SA as is the case for the Wikipedia article. This is why it makes sense to allow for different licenses on Commons, at it has many different images and each might have a different license.
In a few jurisdictions there are sui generis database rights. These cover complete databases - but they do not cover their individual data items. There are licenses built on top of these database rights, and ODBL is the one that is used by OSM.
Unfortunately, CC-BY-SA and ODBL are not compatible. You can not take content licensed under one license and republish it under the other license. Thus follows that using ODBL for Wikidata does not relieve us from *any* of the possible legal issues that are mentioned here due to Wikidata using CC0 as a license.
Having different licenses for different data items in Wikidata, as was suggested here, is not possible, as single data items are not licensable.
For every data item in Wikidata, there will be the possibility to add a reference that supports that data item. The license of that reference has no effect on the license of the data item. This is the same that happens in Wikipedia: in the article on Japan we might and do have references that have proprietary copyright. We might have a reference that has a CC-BY-ND-NC license. But that does not mean that the article on Japan has to be CC-BY-ND-NC. The license of the reference has no effect on the license of the Wikipedia content.
If Wikidata was a collection of databases, like e.g. OKFN's DataHub, then it would make sense to provide for different licenses for each of these databases. But Wikidata does not have any notion of different databases. You cannot take a database and simply upload it to Wikidata. You can do that on DataHub. Wikidata is not DataHub, and does not aim to be DataHub. DataHub is pretty awesome at being DataHub. The goal of Wikidata is different.
Choosing CC0 now allows us to later switch to CC-BY-SA4 if we choose so, which actually does take care of database rights, unlike previous versions. Choosing ODBL would not allow us to do so.
I hope that clarifies the discussion a bit. It is a murky legal area, which has not been much tested yet in courts, so there are some insecurities there.
I hope this helps,