I understand, what I'm trying to do here is separate a good from a nice teacher.
What is a good teacher? McGonagall? Sure. Dumbledore? You bet. Slughorn? He had his flaws, but yes. Snape? Yes, but he was not a pleasant person.
That's my point. Teachers need to be friendly and caring? They do, but if they don't, they still can do a good job. Kinna like Coach Carter type of thingy. He was also extremely unfair, demanding and biased, but he still helped the kids to learn something.
Let's talk qualifications first. Again, apart from Albus and You-Know-Who, Snape was probably the strongest, most experienced teacher in Hogwarts. Snape was the most qualified person to teach DADA. Specially because he was a death eaters and he had an unusual passion for the dark arts. He understood what would drive someone to use a Sectumsempra. Rage, anger, bitterness, yeah, those are all bad things, but it is also the emotion that usually drives the dark arts spell anyways. IF anything the students would have a better perspective from someone who not only knew about them, but probably used or intended to use it at some point.
Then why didn't Dumbledore made him DADA teacher? For one, I like to think that Dumbledore knew Snape better than himself, and despite all of the talk about DA, Snape was better at potions, both because of his life and because of natural aptitude towards potions. He only changed him to Slughorn because he needed an excuse to bring Slughorn back somehow. Even so, he did that right at the time he figured he wouldn't be around anymore, so he needed Snape to be right in the middle of everything. Two, the position was cursed. So much so, that Snape did die.
Despite his personality, Dumbledore made Snape DADA when the students needed the most. Snape was not totally a dick. Again, he was bitter. He lashed out on the wrong people, but he was victim himself. It is not an excuse, but we must remember that this is a man that was ok with You-Know-Who's sadistic perspective at first. If he just became a good person out of no where, that would be a personality ex machina. He was still the jealous, vindictive, arrogant little prick that he had been with Petunia, but that's just the human part of him, the part that we all have every now and then, the emotional side that makes us unreasonable and sometimes irrational. When shit went down, he was more useful than all the other good-doers teachers of Hogwarts combined. Hell, he was the only actual soldier in there. Willing to let go of his own perspective about everything just to fulfill the desire of someone he loves. If you ask me, I'll take that kind of a dick person anytime.
I don't want someone who tells me I'm perfect and everything is going to be ok. I accept someone who is obnoxious and sometimes vindictive, if I know that this person is a 100% loyal to me. That's what Snape was.
And that's what he could teach the kids: talking is easy. The important thing is if you actually have the skill when the time comes. I wouldn't care if he mistreat me because I am a muggle born. As long as he taught me what to give to my friend when he is chocking to death, I'm fine.
Dolores for instance, was an example of a "similarly" nice person who was actually a mean fucking bitch! In real life, people who pretend to be nice like her are easy to find. The point, though, is that she was not only an evil person, she was also a terrible teacher. She didn't want to teach what the kids actually needed to learn. That's just fucking ludicrous. Harry himself had a good response for her, showing that they needed to be in danger, they needed to experience the actual problem.
Tl:dr; Never argued about Snape being a nice teacher. Argue about qualifications. He was more qualified than everybody else, because he knew more and he actually taught them something. The fact that he was mean, was a flaw, sure, but being a bad person doesn't make you a bad teacher.