Over the years, we have had to do our fair share of motorsailing; usually when beating against wind and seas. An infamous motorsailing stretch is the Baja bash in reverse, i.e. when sailing back to San Diego from the Sea of Cortez. Of course, we also motor on other points of sail if there is just not enough wind; often leaving the main up (because we are too lazy to take it down), thereby technically "motorsailing" again. In that case, however, there is of course very little heel.
In the case of strong winds on the nose, however, we usually try to use the engine for "power-pointing" (i.e. sailing closer to the wind than would be possible withouth the engine). In other words, we are usually "pinching" a bit, which brings the vessel back on its feet. So, again, there is not more than let's say 10-15 degrees of heel.
I would even go so far as to say that if you are heeling more than the allowed 15 degrees on the course you really want to sail, you probably don't need your engine in the first place.
The exception could be, of course, if you are using the engine to battle a strong contrary current. This is usually only worth doing for a very short period of time, e.g. when going through a strait, channel or reef passage. If that is going take more than an hour you are probably better off to wait for the current to weaken or reverse.
In practice the danger of heeling too strongly while motoring would seem to be a non-issue most of the time.
Have fun!
Flying Dutchman
