Toastmaster is a great organization and relatively well regarded. At my current company Toastmasters are allowed to bypass some trainings we have that deal with presentation skills.
It is, however one of those organizations that will benefit you more based on what you take away from it rather than from simply having it on your resume. In other words, it'll benefit you quite a bit in the interview in terms of being able to answer the questions and to be composed. It won't, however automatically get you into schools.
No it's not that common among applicants. <5%.