Yes, there are plants that overwhelm others and plants that are too ugly to tolerate but just because a stray seedling shows up in your vegetable garden is no reason to panic. I've noticed that, quite often, plants grow better if they're surrounded or even crowded by other plants. Their leaves shade the ground, reducing evaporation. The roots bring up water from deeper in the soil so moisture leaves more evenly. There's no problem with water running off the surface of the soil. And maybe they just like the company! I'm trying out a group of plants as a living mulch for my vegetables, self-heal, sweet alyssum and moss among them. An experiment, of course, but one with a good possibility of success. More info on this later!
Watercress - a delicious weed in an ornamental stream