They make it simple to get started and even without knowing what you are doing you can easily churn out something that works if it is simple, doesn't change often, doesn't need to scale and deals with small amounts of data.
But similar to http://blogs.tedneward.com/post/the-vietnam-of-computer-scie...
"It represents a quagmire which starts well, gets more complicated as time passes, and before long entraps its users in a commitment that has no clear demarcation point, no clear win conditions, and no clear exit strategy."
RDBMS are the root cause.
There are no major systems out there of even moderate complexity that aren't built on an rdbms.
In theory, it could be used to provide that industrial strength abstraction layer between your Tableau/Looker/etc. and your bajillion weird and not-so-weird (RDBMS) data sources.
That would seem to make sense to me from the point of view of -- I would want my data visualization/analytics-type company to be able to concentrate on data visualization/analytics, not building some insane and never-ending data abstraction layer.
The part that surprised me was that Denodo could allegedly do a lot of smart data caching, thus speeding things up (esp hadoop-oriented data sources) and keeping costs down.
I'm guessing the other data virtualization providers can do similar.