It eventually burns them up, or makes them more prone to problems.

It's like having a CB radio, or a VHF-FM marine radio...and keying it up without an antenna, or a "dummyload"

It'd probably take more than a couple of times...I used to do it routinely. But I think it had a lot to do with a crash I had one day, with perfect batteries, no one else at the field..working swtiches, etc. I never flew the radio again so I can't say with any certainty.