Why recruit such an evil person, with countless crimes and atrocities to her name, if you are GOOD?
Because there's a whole vast of gray between 'moral, good pure hero' and 'murderhobo evil'. If you're playing a good hero character you probably don't want Minthara, Astarion and Lae'zel in your party. If you're playing an evil character you probably don't want Wyll, Karlach and Halsin in your party.
The ability to knock her out and still save the Grove, and then recruit her at Moonrise later on is meant for the large number of players who want to play in the wonderful gray areas between those two extremes. You're not forced to recruit her, you can still kill her. You can still raid the Grove too, if you want.
Also, you know, gotta catch 'em all.
