Is it useful to do an MBA after PhD? I have heard of people who have gone on to pursue an MBA after their doctorates. What are the advantages/downsides of this? 

Wouldn't the companies recruiting them after MBA value their doctorate experience less compared to the experience gained at an MNC? Is there a risk of them being valued as "failed" doctorates?
There are several cases where PhDs do an MBA. Some of them are:


Bored of research as a whole.
Irritation with academia and poor scope in Industry.
Want to earn money.
For furthering responsibilities at the university where one is a professor. If you do some looking around, a high amount of Deans to have an MBA along with a PhD.