1) You’ll be free to mix and match brands and types of knives. Japanese versus German. A Shun santoku with a Wusthof paring knife. Often brands have their strengths (or weaknesses) that you can use to your advantage or to simply give yourself a change of pace. The most striking example would be to mix lighter, thinner, Japanese-style knives with heavier, thicker German-style ones. You might prefer the lighter feel of the Japanese as your chefs knife, but find it too flimsy or slippery as a paring knife. Or vice versa. (Photo above: One of KKG’s knife blocks which is pretty far from a typical set.)
2) You won’t be tempted to settle for quantity over quality which is short-term thinking and shouldn’t be done with a tool you’ll probably use 2 or 3 times a day for decades. Better to have one primo blade than two or three passable ones. Plus, it’s easier to care for and keep in tip-top sharpness a few knives rather than a bunch.
3) You’ll spend less money by avoiding the common consumer trap of getting carried away with the opulence of it all and buying more than you really need. Nuff said?
4) You’ll be able to customize exactly what types of knives you buy to suit your specific needs and tastes. This is slightly different that Reason #1 and is more apropos when you’re considering larger sets. For example: Say, you never eat bread. Why pay $80 and up for a bread knife? Or say you’re a couple and both of you like to do prep that involves chopping. But the sets you’re looking at only come with one chef’s knife (and a utility knife which is too narrow for most chopping). Or say you know you prefer a shorter 3 1/2-inch paring knife, but the only size that comes in a set is a 4-inch.
5) You won’t end up with extra knives you never use that sit around taking up space in your knife block for decades. Yes, decades. (I speak from experience—I can probably count on one hand the times I’ve used my 6-inch utility knife.)
Reason’s #1 and 2 are probably the most compelling, but they all have credence. Best thing to do is to mull it over and not automatically assume that a knife set is the best solution. Don’t be swayed by the sheer grandness of kitchen knife sets! (And see Three Kitchen Knife Sets I Recommend for specific tips.)