I believe that excellence should be rewarded, and that teachers routinely go unrecognized and seriously under-rewarded in the monetary sense. 
But do the people who advocate merit pay as an “incentive” actually think that somehow teachers are “holding back”?  Do they imagine that we’ll “teach harder” if we are offered extra money?  If so, they have some really weird ideas about what motivates teachers!
There’s currently a popular mythology about “bad teachers” who need to be gotten rid of.  This is an ugly, distracting distortion.  It’s true some don’t live up to the calling, but the vast majority of us are here because we care, and we are doing the best job we know how to do. 
If you truly want to help educators teach children better, give us adequate funding across the board—with better salaries and professional development, and with quality materials and equipment.  Make sure our students have reliable access to good health care and nutritious food, and that they have safe places to live, in supportive communities where they all receive encouragement to learn (think “Harlem Children’s Zone”).
Sure, it’ll be nice for a few of my excellent colleagues to get more money because they teach well.  But don’t expect “merit pay” to make a discernable difference in outcomes, because it won’t.