From a branding point of view the bad thing is that when you say the name of the domain over the phone you always have to say bla-hyphen-bla.com which is a pain.
What if its one of those names that its rare to even find hyphen. take the chance?
Buying the domain is the least expensive thing you can do. Building a brand and investing in launching a site on the other hand can be one of the most expensive things you can do. An amazing site can do well in spite of a bad domain name (example: pinterest!) but a well known domain doesn't guarantee a great website or brand.