Well apart from the definitions, you give up a lot of practical stuff that goes along with it.
For example, if your software doesn't meet the Debian Free Software Guidelines, then it won't be packaged in the main repos. And that has downstream impact on other distributions such as Ubuntu.
Not just "a definition", it's labeled Freedom Zero. The ability to run the program, for any purpose.
http://www.gnu.org/philosophy/free-sw.html http://opensource.org/docs/osd#fields-of-endeavor (The OSI's open source definition is derived from Debian's Free Software Guidelines, which includes the same prohibition against licenses banning fields of endeavor).
Well apart from the definitions, you give up a lot of practical stuff that goes along with it.
For example, if your software doesn't meet the Debian Free Software Guidelines, then it won't be packaged in the main repos. And that has downstream impact on other distributions such as Ubuntu.