"Any time SBD is over 1$ you should sell on the internal. Any time it is under 1USD you should use convert".
So if sbd is under 1$ it is fine to use convert, but as you say the orice could jump during conversion. I guess it's just a button for lazy people or those who don't want to learn how to use the market!
The 3day wait time locks up the price and the SBD, so not a good idea to use it at all. The market can move during 3.5day period and that isn't fun with larger amounts.