At some time or another, most of us sail on the jib alone, for the same reasons as you. Sometimes it's too hot to work that hard, or you're too tired. But the boat doesn't perform well when you sail on the jib alone. The mainsail helps the boat tack and sail to windward, and, absent the mainsail, the boat wants to sail downwind. You can nurse it to windward, but it isn't efficient. When most of the driving force is forward of the keel, the sail tends to pull the bow off downwind. The only way you can get the boat to go to windward is if you can get its speed up enough so that the keel generates enough lift to overcome that tendency. More often than not, I've ended up exerting more energy trying to get the boat to go where I wanted it to, than I would have exerted if I had just raised the mainsail. As a result, I don't sail on jib alone very often.
I don't worry too much about stressing the rig or hull when sailing on jib alone. Most rigs are strong enough to tolerate it, at least until the wind becomes really strong, and, when that happens, the boat will become pretty unmanageable without a mainsail, and you'll likely furl the jib and use the motor.
Last edited by Sailormon6; 08-14-2006 at 10:45 AM.