I could not disagree more. If you care enough to knit something, put the time and money into it, why wouldn't you want it to look good, inside and out. It takes very little extra effort to join and weave in new yarns. Besides, knots could come undone unless the sweater felts a little. I believe that if you are proud of your knitted project, be proud of both sides.
If someone wants to see the inside of my sweaters I won't say, "sorry, but I don't show the other side of my work."
To prove my point, here are a couple of sweaters, designed by Kaffe Fassett, that I've knitted. I'm happy to show both sides and make no apologies. You can click on the pictures to enlarge them.