You've almost certainly heard this before and heard the reasons, but I'll repeat it because so few people seem to believe it: A dull knife is a dangerous knife.
Danger Will Robinson!
A dull blade requires more force and provides less control than a sharp knife. I can cut perfect 1/8-inch slices of tomato with any knife I own with almost zero pressure. The knife seems to melt through the tomato like a hot knife through butter. For something tougher like a raw sirloin roast only a bit more effort is required.
I will confess, I occasionally cut myself when my attention is distracted. But I often don't even notice the cut until I see blood - it's painless. And I've cut myself far worse and with serious pain using someone else's dull knives. In fact, when I cook at my parent's house I always use my own knives (as a chef I keep a set of knives in my car) because I got so tired of cutting myself. My mother's knives are as good as mine, but are never sharpened unless I do it.
You’re far better off with a cheap but sharp knife than an expensive and dull one.
But sharpening knives needn't be an ordeal. There was a time when I would sit down every six months or so in front of the TV with all my knives and a whet stone and spend a couple of hours making them razor sharp. I don’t have that kind of time anymore and besides, the knives I use most need sharpening more often while those I use less frequently can easily go a year.
A sharp knife is one with two precisely aligned metal planes (the sides of the knife) that come together to a microscopic edge. The worst enemies to this edge are the things you slice and, by far, your cutting board. Because the edge is microscopic it is also delicate. A hard squash like butternut or even a potato can warp and bend the edge, ruining the alignment. And when you cut through something and reach the cutting board the damage is worse.
So the primary cause of a dull knife is a misaligned edge. The solution to this is a sharpening steel. A steel realigns the knife's edge. Whenever I pull a knife from my rack with my right hand I also pull my steel from the rack with my left hand. The knife gets a half dozen strokes on each side before I use it. I don’t straighten it again unless it goes back in the rack, at which time, if I use it again, habit makes me grab the steel and straighten it again.
The two knives I use every single day remain safely sharp for 3 months and in the six years I've had a knife kit in my car only my chefs' knife has been sharpened - twice.
Which brings me to sharpening (as opposed to aligning with a steel). Eventually you will wear that microscopic edge down and the knife will truly become dull. When that happens you need to remove some metal. That's what something like a whet stone or the Chefs Choice electric sharpener does.
The goal in this case is to remove enough metal to restore the precise alignment at which the metal planes meet. After using a sharpening device you'll find a fine metallic powder is left behind (you should never see this when using a steel).
My Daddy taught me to use a whet stone when I was a kid and as I noted above it stood me in good stead for many years. I still have an Arkansas stone and use it on my favorite knives about every 18 months to completely refurbish the edges. But these days I rely on a Chantry knife sharpener most of the time (review here).
Depending on usage sharpen your knives every three to 12 months. But use a steel to straighten the edge every time you use them. You'll work less in the kitchen and be safer.
By the way, to judge the sharpness of a knife, hold up a sheet of paper by a corner with two fingers. Then draw the knife though an edge of the paper. If the knife cuts the paper it's sharp, if it doesn't it isn't.