Harnesses can be good if a coach knows how to use them properly: For the skater who is working on 'feeling' the rotations in the air, etc. It should be a gentle assist on landing, not a whole-sale "hold'em'up" type thing. It can be a helpful confidence builder, or a "crutch"- it's like training wheels on a bike = you get your balance with them, then take them off ... the problem is the kid who is using the training wheels FOR the balance, and doesn't have it without them, so, when they come off ... they haven't learned anything except to rely on having that balancing thing.
Because straight line harnesses are "artificial" - who takes off like THAT! - they don't help with jump entries or exits out. A pole harness, with a trained operator, is good - they are able to adjust the support and landing, can set it up with a real jump entry, and, because they are right beside the skater, they can correct and observe. A good pole harness operator can "jump" a 6'2" skater into a triple axel ... that's how little actual "lifting" is done.
Our pole harness guy is a specialist who comes in by request, and he's excellent. There is another one who is trying to learn how to use one ... who is basically fairly ineffective ... as he simply jerks them off their feet for takeoff, then, thunks them down on landing. Our coach is an expert on using the straight line harness, and does it with effect where necessary, not as a regular thing: the jump technique is taught properly, then, the skater is "moved up" the level to feeling the additional rotation with the harness; the skater is expected to hold the landing with only minimal assist from the coach, otherwise, the jump isn't ready to go in the harness. If it helps in perspective: I've seen her small 5 foot height jump a 6'1" skater into a triple axel on the harness with no problem at all ... and we're looking at someone who, well, you're not 25 years old when you've been coaching for 40 years - so you can imagine how little "lifting" or "holding" she's doing.
Lots of coaches use the harness as a "fun way" to keep kids engaged, and, frankly, it's of little jump technique benefit. Some kids take years to get doubles; it can be caused by a variety of factors, and can be jump technique, balance and coordination issues, skate issues, growth spurts ... etc etc etc. A harness is just a helpful aid in some situations, certainly not a cure-all ... I like FigureSpins' suggestions ... she's absolutely right ...