I completely understand running the 20 inch bar for limbing , I did it and very much liked standing more upright and the reach it gave.
but limbing with the larger bar isn't putting the stress on the saw that that burying the hole bar in a log bucking 24+ inch in diameter rounds and repeating it 10-12 times until your out of gas , gas up then repeating again.
there is a place for the longer bar in limbing and occasional larger rounds .
dealers selling over bar-ed saws get away with it frequently because few people really work their saw , most are not going to cut round after round of 24+ wood
It's up to the buyer to do there research on anything they buy and not trust the person selling what ever it is. It's the same with anything you buy. Research, research, research, then start shopping. And with the internet, it's so easy to do these days.
I ran a ms 290 with a 20'' bar (still do) witch is not a high dollar saw and is the maximum bar recommended for it, I run it hard for hours running tank after tank cutting 40" rounds only stopping long enough to sharpen the chain and refuel, in 100 degree heat.
I do have a ms 362 that I sometimes run a 24'' bar to get through some of the bigger wood but I generally run a 20'' on it as well. And in my opinion the 362 doesn't run a 20'' bar any better then the 24'. I have also found when the bar is too short for the wood your cutting that it is harder on the saw. If you can have some of the tip sticking all the way through, it cuts faster and better then when the tip doesn't go all the way through. After running a saw for a while you learn there is a sweet spot (correct angle) that a saw cuts at it's best. You can feel it when it hits that sweet spot and the saw really starts sinking into the wood sending out a shower of long thick stringy chips. You soon learn how much pressure you can put on the saw before it starts to bog down (sweet spot) You just have to learn the limits of the saw bar combo's ability and don't over work it.
I just bought a new ms 261 c witch is very close to the same size as the 290 but with a lighter power head. Right off the bat I could tell with the lighter power head, I have to put a little more pressure on it to make it cut the same as the 290. The heavier power head works better because I can just let the weight of the saw do most of the work. Basically letting the saw cut under it's own weight without having to add any more to it by forcing it. Again hitting that sweet spot where the saw cuts its best.