Wikipedia Signpost is a publication of the English Wikipedia. It published a piece about copyright and Wikidata and it suggested that a more restrictive license would be fine. Their problem: others benefit and do not need to acknowledge Wikidata as a source.
For me the most important thing of our work is that it is used. Everything we can do to make our data used more increases the value of our data, This is best achieved by refusing to put any restrictions on our data.
One argument for another license is that "it recognises the labour that goes into maintaining the data". The question is how to recognise this and why. Every data point has its own history both for the property and for the data and as a consequence it is the database that you refer to for the attribution. For human consumption it is the label that gives Wikidata much of its relevance; giving tribute to the people who add labels is as relevant.
Data is mostly generated in an automated or semi-automated way. I would not have over 2 million edits if all statements I added had to be done by hand. With StrepHit, a tool that retrieves facts from authorised sources, data gathering will become even more sophisticated, reliable and complete. The link to personal glory in attribution becomes very much absent.
Wikidata will become increasingly rich in references and tools like StrepHit will ensure the quality of such references. Wikidata is already very rich in references to other sources of data and it is why Wikidata will evolve into a resource for comparison with the data in these sources. These other sources may opt to adopt or report and the same is our option. Comparisons allow us to research the issues that exist with the data we hold and these comparisons will become highly automated and intelligent.
My point is very much that Wikidata is not a glory project. Our data is incomplete and immature and in several ways more ambitious than what a Wikipedia aims to do. Wikidata can include the ambitions of a Wikipedia up to a point. To realise its own ambitions, becoming a valuable and valued resource in the web of data set, it is important to be as open and available as we can be. A license that does not restrict is one of the underpinnings. Moving towards a more restricted license will only create a morass of uncertainty and doubt. It will bring us no benefit.