Okay, to build out an answer here, (along with some knowledge of melee combat), there are some things that need to be laid out first.
1: While lightsabers are weightless, they produce a gyroscopic effect...which means they resist changes in motion...so it takes force to get them moving, and then takes more force to stop them moving once they get going. This is generally cited as the reason why lightsabers are so dangerous to the user, and why Jedi are normally the only ones who use them. (Once you got them moving, it could be hard to get them to stop)
2: A longer blade produces more gyroscopic force, this is why the 'lightsaber shoto' was favored by some, because it could be moved more easily and quickly than a full length lightsaber.
As a result, this means that fighting with a dual-phase lightsaber would be the equivalent of swapping between a one handed longsword, and a greatsword at a moments notice...only worse, because a greatsword isn't twice the length of a longsword.
So, here's a breakdown of what happens when you flip the dual-phase switch to extend the blade: Your blade doubles in length...it also doubles its gyroscopic force, so if it is already moving, good luck stopping it. This has the potential to be extremely potent in combat, but would be hard to master, and limits flexibility of the fighting style. For example, you could start with the blade short, extend it to increase its 'momentum', then re-shorten it before you try to stop it. This would let you hit like a warhammer without having to start or stop the warhammer's motion.
Unfortunately, this also means that any time you want to change the direction of the blade (which happens a lot) you have to hit that toggle, and are limited by the switch delay of the blade changing length. A brief delay like that can cost you your life in a battle that high paced. This is further complicated by how a dual phase lightsaber could work. Corran Horn's dual-phase saber had its length switched by twisting the handle, this requires two hands and is more time consuming that flipping a switch.
As a result, while this makes the weapon potentially very powerful, and quite good at surprising your opponents, it would be very difficult to master. You would have to learn to fight with a weapon that could double or halve its functional momentum at a moment's notice...and it can be hard enough to work with weapons of different mass, even if they aren't spontaneously changing in your hands.
So, in summary, a dual-phase lightsaber could be a very powerful weapon, but very difficult to develop mastery over, and even more likely to maim you while you are figuring it out. All of this together is why Luke dubbed dual-phase lightsabers 'something of a fad' for Jedi, and Master Skarch Vaunk dismissed them as 'only good for surprising an opponent.' In short, it seems that they were simply considered more trouble than they are worth.