I've not "done the math", but venting slightly warmer water vapor from a tower seems less ecologically damaging than releasing warm water back into an ecosystem that can't handle it, by diluting the waste energy over a wider area.
In the end it's about energy dissipation; that heat's got to go somewhere, and there's only so much you can recapture with cogeneration.
Drawing on ground water and releasing it into the atmosphere isn't great if the demand exceeds the aquifer's ability to recharge. Discharging heated water into a retention pond might allow it to recharge the aquifer (although some would evaporate), and form a less open loop. Or, discharging it into sewers might offset fresh water needs at a downstream treatment plant, and the temperature differential may or may not be important. Sometimes, reclaimed water from treatment plants can be used for cooling as well, and that's better than pulling fresh water directly.
In the end it's about energy dissipation; that heat's got to go somewhere, and there's only so much you can recapture with cogeneration.