Sailboats are much more stable when their sails are full and drawing; dropping canvas to motor in rough conditions can make for nasty boat motion. And if you always do what's easiest, will you be able to get off that lee shore on sail alone when -- not IF, WHEN -- your motor croaks? Learn to claw. A motor is no substitute for seamanship.
As for motorsailing ... please. In very light air, motoring ensures the apparent wind is straight on your nose, no matter what your heading. Fall off and the 'wind' falls off with you. The sails are just added resistance, helping not at all. If you decide to motor, fine -- motor. There are times when that is the best, safest, and perhaps only choice. If you want to sail, sail. I have seldom seen a boat motorsailing intelligently, tho at Catalina Island I saw a lot of sailboats motorsailing in perfect 10kt winds, with no regard to sail trim or heading. Most obnoxious was a (reasonably expensive) sloop heading DDW from Two Harbors to Avalon -- in 10 kts, remember -- with the motor running, headsail furled, mainsail up -- and boom close hauled.
The boat was plunging and lurching violently as it performed a series of mini-jibes, and we were honestly waiting for it to knock itself down. In ten knots, with mild seas. Odd you say you aren't sure why more people don't motorsail; four-fifths of the sailboats at Catalina were motoring, sails up or not.
I also have philosophical issues with the convenience argument. I'm neither a Luddite nor a purist; I do, however, wonder about people who own sailboats but motor them most of the time because it's easier or more convenient. Sailing is about doing things the hard way, isn't it? There's no rational argument for sailing these days -- we must choose this anachronism because it's tricky, and esoteric, and requires us to learn things about the sea, the boat, and ourselves. ANY idiot can drive a powerboat. It takes a SPECIAL KIND OF IDIOT to make a sailboat go.
And as Boasun says, sailboats make lousy powerboats -- underpowered, non-planing, bad-steering, prop-walking cows. If you prize the benefits of motoring a sailboat, you'd probably be even happier on a purpose-built motorcraft. It's like putting a gasoline motor on a bicycle: it's not as good as a motorcycle or scooter, yet you've just nixed the primary purposes of riding a bicycle: environmentalism, fitness, and a feeling of accomplishment. Oh yeah -- sailboat motors have pretty short lives, so using them casually will lead to rebuilds or replacement every five years or so, which may cost far more than the boat is worth. Sails cost money and wear out too, so motorsailing (which strains the motor AND flogs your sails to kleenex) is doubly expensive.