Pop quiz: when was the last time you sharpened your knives? (And no, using a honing steel doesn't count.) If it's been longer than six months (and even that's pushing it), don't even think about using your chef's or paring knife to slice tomatoes. The blade won't be sharp enough to smoothly slice through the tomato's skin without damaging its delicate flesh. A serrated knife, like this prep knife or a bread knife, is a much better tool for the task, as the scalloped edge diffuses the pressure on the tomato. Plus, these types of knives keep their edge much longer.
For the same reason, pull out a serrated blade when slicing other delicate produce like plums, nectarines, and peaches.