If you put 20 single-engine light aircraft pilots in a hangar and ask them about slips to a landing you will get 21 (or more) opinions. Several years ago I was asked by an FAA inspector to help them clean up a mess left over from an "old school" designated examiner who taught that "real pilots don't use flaps". This guy had over 35,000 hours of dual given and had been a designee for something like 40 years. The problem was that, for years and years, his flight school used aircraft that didn't have flaps - Piper Colts, Champs, and Cessna 120s. Finally he updated his fleet with Cessna 150s, but his teaching techniques didn't include the use of flaps. This guy would train all of his students himself, but prior to the checkride, one of his staff instructors would sign off the recommendation. The checkride would be given by "No Flap Frank". After a while the FAA got wind of what was going on and pulled his designation. I was asked to provide a some remedial training to a few of his graduates.
Slips do have their place. However, heavier aircraft, regardless of whether they have one or more engines or are piston or turbine powered are best flown using flaps and power in the pattern. That being said, a while back at one of my 6-month recurrents at FlightSafety, my sim instructor announced that I had contaminated fuel and failed both engines ten miles from the airport. With both engines out, there was no hydraulic power for the airbrakes and the gear had to be blown down. I used flaps and slips to make the landing. As a CFIG it sure would have been embarrassing not to have been able to put the airplane on the runway. Does a pilot need to know how to do slips. Absolutely. Just make sure you heed the warnings and cautions in the POH.
'Sled