it's basically the same argument as for Zombies, the proper slow moving unending zombies like in World War Z the book and not the atrocious World War Z the Film that tried to make Zombies weird and more scary instead they destroyed the fear.
a Walking Skeleton/undead/Zombie has many pros and a few cons, they are already effective, you don't need to do more with what your have, they are already fantastic
Cons, (for classic skeleton soldiers)
They are weaker (but magic could easily overcome this)
They are slower (Again magic can overcome this)
These cons can be overcome quite easily with the magic that your necromancers use, they already are using some to make a skeleton walk in the first place so why not?
Stopping to eat and drink
They don't need feeding, they just are. living people need to eat and drink, skeleton soldiers need neither, they can keep going and going, while the humans will need to stop eventually to eat/gather supplies/collect water, the skeletons keep coming never ending
The don't need training, at least not normally, they just come to life and start attacking, yes a trained swordsman will be able to fight off a few skeletons, but ever tried fighting off more than one attacker at the same time, it doesn't take the same amount of skill, it takes exponentially more skill to be able to protect yourself from multiple angles while at the same time attacking when the opportunity arises. try it with some friends and broomsticks, just bring some ice-packs, as you're going to get hit a lot... an untrained human will find it hard to fight off a a single untrained skeleton, they both just need to get unlucky and its goodnight, mix that with my next point and it gets worse for humans
They have near infinite endurance, and don't need sleep, the longer a soldier stays awake the less effective they are, that is why the training is so important to extend their endurance, and minimize this loss as long as possible, but if the enemy keeps coming slowly but surely, the soldiers grows weaker over time but the enemy doesn't. a man can swing a sword for some time and be reasonably effective, but to effectively block another sword continuously for more than a minute, it gets tiring very quickly, each block will be weaker, each strike softer, each counter slower... Skeletons don't have that issue, they just keep going
they don't get bored or demoralized, and stop fighting, most large scale battles ended in one side being routed and running away, and the other side rarely chances them down, why risk more loses when the enemy is running away and scared out of their minds, but skeletons don't have this issue, they keep attacking, they won't run away, they won't get tired or bored chasing down the runners, they keep going they keep chasing
Its easy to make a huge army of them, skeleton soldiers rarely fight one on one, they always fight as a horde, and it is that horde aspect that makes them both scary and effective. what does it matter that a human can stand against a few skeletons and that an dozen soldiers could kill a 100 skeletons when there's 1000 more over the hill, its never ending, a constant struggle constant fear constant attack, no sleep no food never time to at least... just constant miserable fighting
At last but not least... they are really really Scary
Most people can deal with the sight of a skeleton, we've all seen one on TV, in a science classroom, whats it matter, well... those don't move, they don't attack, they aren't trying to kill you, this has a massive mental effect on someone, something is trying to kill me, it causes fear... fear that is only added to by the above factors, hence my repetition in the above factors of the constant unending assault. eventually it starts to build and compound on top of each other, and eventually the fear gets too much, going up against an army of the undead/skeletons would lead to a lot of suicides, which in turn could lead to more bodies for the necromancers
Think about finding a soldier that has hanged himself, he couldn't take it anymore, this was your fellow soldier, your friends. you give him his last rights etc, pray for him, then a necromancer turns up and resurrects him, but now he's not your friend, not that person that you ate with, trained with, fought side by side with, now suddenly, he's your enemy, he's trying to kill you. how badly would this traumatize people?
Add this onto Tim B II's suggestion of the increased disease factors... and well... need i say more?