Boomer
Have had brief experience with both in-mast and in-boom mains. On one Newport-Bermuda race, we tried, with a full racing crew, to get the most out of our in-mast r/f main on a semi-custom Bristol 48. Out of about three 24-hour days, IMHO we got the sail set well and performing for about a half hour. With no roach, no battens, it was miserable. Add to that the worry about something going wrong at the top of the mast and then not being able to get to it because it was INSIDE the mast... no thank you. The summer of 2003 we were invited for a cruise with some friends who have an in-boom system. It was like night and day. Furling the sail lowers its center of effort considerably -- exactly what you want when you want to reduce sail in heavy wind. If something messes up, it's in the boom, where you can get at it. As you point out, you can also lower the sail entirely and simply tie it to the boom with sail-ties if you need to. For light air, you have as much roach as you need to boost sail area and performance, plus battens help the sail last longer by reducing flutter in the leech. That said, we have jiffy reefing on our boat.