First, how comfortable is the golfer in hitting hybrid clubs? I would say 20-30% of our golfers going through a fitting have trouble hitting their hybrids consistently. Usually, it is because of the fact that the hybrids are not very well fit to them in terms of shaft flex and weight. Unfortunately, sometimes this becomes a mental block that is difficult to overcome. We get past that by having the golfer hit a hybrid that is matched to their swing and compare it to their existing hybrid. This usually gets rid of the fear factor!
Second, how well does the golfer hit their long(er) irons? This is a little bit of a trick question as most golfers want to keep a longer iron, like a 4 or 5 iron, in their bag to get out of trouble if they are under a tree branch. But that is a total waste of a club. I can easily knock down a 7 iron if I have to punch out. If the golfer only hits the longest iron in their bag consistently, 50% of the time, it is time to switch it out for a hybrid. This is especially true when the golfer is trying to hit the long(er) iron from a poor lie, the rough, or off the tee on a long par three. The hybrid will be much more forgiving and consistent, especially if it is properly fit!
Be careful trying to replace a fairway wood with a hybrid. Sometimes our golfers are very comfortable with their hybrids and want to try to replace their 5 wood with a 3 hybrid (18 or 19 degree loft). Not a bad consideration, but only if the golfer has enough club head speed to get the 3 hybrid in the air. Normally a 3 hybrid will not go the same distance as a 5 wood because the 3 hybrid has a shorter length. So be a little careful trying to go with too low of loft in the hybrids.
In general, a properly fit hybrid will provide a lot more versatility than long irons in nearly every golfers bag. The only exception to this is that hybrid club usage on the PGA Tour is actually declining as more players are opting for driving irons when they need a more forgiving long iron. For me, my trusty 4 and 5 hybrids may be joined soon by a 6 hybrid!