And sometimes it's not so much to rent the basements out, but to provide a second living accommodation. Sometimes it's for a relative. I think there's a growing trend now for younger folks (who maybe have graduated from school and then returned home and end up living in the basement) to have everything they need (bedroom, bathroom, kitchen) downstairs.
This question has alot of variables. In some areas we see new construction/ and secondary residences implementing generational living options (mother in law quarters) is a possibility, a secondary kitchen for guest use, entertainment function, rental income, business operations. The sky is the limit!
Are there benefits and drawbacks...absolutely but I believe its all in the eye of the beholder and a matter of what works for your living uses. Here is an interesting forum on the subject to project further the uses people might have for a second kitchen.