It would be great if one log could be used for multiple callsigns, e.g. on my netbook when I am travelling it would be nice not to have to set up a new log each time I need to sign with a different callsign (M0TRN, M0TRN/P, MW0TRN, MQ0TRN, OV5T, OV5T/P, F/OV5T etc.)
Sure it could be done with QTH profile and just ignore the fact that the program doesn't know which call I used, but there should be a "my callsign" field in the profile so it would still work with LoTW uploads etc.
Also if this were done, and enabled in preferences, the "My callsign" field could be shown in the list of previous QSO's. This would allow having one master log and e.g. I would be able to say "I worked you last year when I was in Denmark as OV5T", or "Worked you with the Queen's Jubilee special prefix" etc. and be able to share call comments over many logs. I know it's probably only a minority of people who use a number of different callsigns but for us who do it would be an extremely useful feature :)