I know SDRs have real and tangible benefits right now as receivers (on HF, and elsewhere), but what specifically is there to gain by using a SDR as a transmitter for HF (specifically, the 20, 30, and 40 meter bands) if you're an American ham subject to FCC rules? I love the idea of SDR. I think it's unquestionably the way radios ought to be, and in another 20 years or so, almost certainly will. But I'm struggling badly to find of any concrete advantage to using one as a HF transmitter today. For VHF, UHF, and microwave bands... sure, there are plenty of good and obvious reasons to want a SDR transmitter... like cool, exotic, wideband transmission modes that would be impossible or impractical with today's normal rigs. But on HF, as far as I can tell, pretty much anything a SDR transmitter could uniquely do that a conventional rig can't accomplish with a soundcard + SSB is, as far as I know, prohibited by the FCC. Am I wrong? Is there some cool, bleeding-edge experimental digital transmission mode that's legal to use on HF, has concrete advantages over modes like MT63, THOR, and Olivia, and actually needs a SDR transmitter? As an alternative, how viable is it to use a SDR as a receiver, together with a conventional HF radio as the transmitter (just ignoring the transceiver's audio output, and using some kind of PTT-triggered automatic coax switch to connect the antenna to the transceiver a fraction of a second before transmission begins, and switch it back to the SDR a fraction of a second after transmission ends)? Is there some huge drawback I haven't thought of, or would this actually be a sane & sensible compromise that doesn't really have any significant downside besides the loss of bragging rights & desk space?