Thoughts On Merit Pay

So the new trend in education is for politicians, etc. to demand that teachers get "merit pay". i.e. a teacher's salary would depend on how well his or her students do on standardized testing. Which, I guess, if you have no idea what you're talking about, makes perfect sense.
I get that there are crap teachers out there and we've all had one or two or five. I get that there should be some way to get those teachers out of schools. I'm just not sure merit pay is where it's at.
Most detractors will say that merit pay would keep the good teachers out of the bad schools. Schools like mine wouldn't have any decent teachers because they could get paid better at a school where the scores were more likely to be high. The argument against that is that teachers would be judged on a how a student's scores improved from year to year, not necessarily a good score, just better than before.

Well, that makes sense. However, these are my issues with that whole scenario:

1. I got a new kid two weeks ago. We take state tests at the beginning of March. To which teacher are his tests attributed to? I've barely taught him, yet he will take the test in my room. I've gained and lost almost 50 kids this year. What happens to their scores? What about a kid who I got 2 months ago? 3?

2. I teach Social Studies, which is not a tested subject. What am I "graded" on? Reading scores? What about the gym teacher? What's he graded on?

3. What if the Reading teacher quits half-way through the year? Or the kids always have a long-term substitute in that room? What if my principal doesn't like me and puts me on a team with a whole bunch of crappy teachers? I only get them 1/4 of the school day, how can you judge me based on what happens the rest of the time?

4. What if the teachers from the year before somehow inflated test scores? (This happens a lot when we see 5th grade scores and then we actually teach those kids...how can you read on a first grade level but get Advanced in both reading and math?) Can I petition someone before those kids take the test to let it be known I think their previous scores are inaccurate?

5. What about teachers who teach Advanced kids? Are they going to be punished if their students, who were previously reading 3 grade levels ahead, aren't now reading 5 grade levels ahead? If they already got advanced on the state tests, there's really no where else to go.

I think there's this whole idea that if you've been in a classroom (which everyone has) then you know what it means to be a teacher. And there's clear ways to measure how good or bad a teacher is. Unfortunately, it's not that simple. If it were simple, it would have been in place a long time ago.

Don't get me wrong, I would LOVE to make more money when my kids do a good job. I just have my doubts as to how it would be possible for that to happen.