To add the post above me:

You chose not to teach new players. You also chose to limit the amount of practice they were able to get. Now those bad DPS are going to get slotted into another party where someone else might quit on them because they're "terrible". See how this cycle creates horrible DPS?

Take the time to teach people. You'll be amazed at how much people can improve with simple instruction. Then those people might pass it on, and the community as a whole reaps the benefits. Dropping a raid or dungeon after a wipe or three simply means people will never learn their job properly because next time, maybe they'll be slotted with an excellent DPS and essentially get carried through content because they lucked into a party where they're the only one that sucks (through no fault of their own, I might add). Even tanks can be carried by a pair of excellent DPS and a stellar healer. Everyone should be committed to helping others improve, it's the only way you're going to curtail the amount of bad players you see in DF.