Alright, I didn't want this, but since it seems likes it's going to happen anyhow, let's actually have the discussion. (Note: Throughout this, I'm going to be talking about mid and high end gear, I'm ignoring both the 50 dollar bilt textiles and the lightweight calfskin leathers.)
For a very long time, textile couldn't even begin to approximate the abrasion resistance of leather. Over the course of the last about 15 years, and especially in the last 5 or so, textile has been making great strides to catch up. That being said, even today's textiles can't really match a heavy or superheavy weight leather for abrasion, best you're going to get is about a third as good. This is largely why people on the leather side of the argument are so adamant that leather is superior.
The problem with that is that abrasion resistance isn't everything to consider when you're talking about protective apparel. You've also got puncture resistance, cutting resistance, tearing resistance and bursting resistance. (Leaving aside the matter of hard armor, since theoretically there's no reason leather armor can't be just as well provided with hard points as textiles, though generally it isn't.)
Puncture resistance is fairly straight-forward, high end textiles and leathers both offer approximately the same resistance (though leathers tend to slightly edge out textiles in that regard.)
Cutting resistance is better in textiles, since one you pierce the leather, it's resistance drops down significantly, whereas textiles maintain almost the same resistance throughout the cut.
Tearing resistance is phenomenally better in textiles than it is in leather, both have similar resistance to the start of the tear, but once a tear has begun, textiles keep almost the same resistance to the continuation (minus the additional leverage granted by the opening), whereas leather has almost no shear strength once a tear is opened.
Bursting resistance is whether or not the garment itself will hold together during an accident or whether the seams will burst. This one is the trickiest to talk about, because you have to talk in generalities about construction, and there are of course exception in each direction for each side of the debate, moreover, sadly you can't just let price be your guide in making the determination, as some very expensive gear on each side performs surprisingly poorly. With all of those caveats, though, generally speaking textiles have better bursting resistance than leathers do. This is for a few reasons. First, See above about what happens to the shear strength of leather once it's been pierced, in this case by the needle. Second,, it's bloody hard to work with leather, thus you wind up with manufacturer cutting corners by doing things like using single row interlock stitching rather than the double row lock stitch that you commonly get in textiles (Again, I'm talking generalities here!). Third is that textiles wick moisture away from the thread after getting wet, whereas leather holds the moisture that wicks down the thread into the hold, which weakens both the thread and the leather.
Now, having talked about the physical properties of leather and textiles, let's talk briefly about uses, because that's something I commonly see people ignore.
When you're track racing, you're riding at high speeds in a controlled environment. If you go down, you're going to hit the ground, slide along good asphalt (and maybe gravel/grass) for a good long while, and hopefully stop before you hit the barrier, but maybe not. If you (and another rider) are exceedingly unlucky, you might get hit by another bike. In that scenario what you want is very good abrasion resistance for the slide, and hard points for barrier and/or the other bike and, of course, good bursting resistance, because no garment will do you any good if it comes apart on you. Leather makes great sense for this.
Street riding is a whole different ball game. You're riding at much lower speeds, in an uncontrolled environment. You don't need the same abrasion resistance, because you're not going to be sliding along nearly so long (If someone is doing 140mph on the road, I hope they're not wearing any gear at all, we need them out of the gene pool.), but who knows what you're going to hit. Maybe you'll go through a windshield, maybe you'll go into a stop sign, maybe you'll run into a barbed wire fence, maybe you'll slide through rocks (as opposed to gravel). In those scenarios, the other forms of resistance become vastly more important, and that's where textiles shine.
Bah, this wound up being much more of a wall of text than I wanted it to, sorry about that, here, have a:
TL;DR: This is not a simple debate, Textiles and leathers each have their strengths and weaknesses, what you wear should depend on what you need, not a grand sweeping statement. In general, leather for track racing, textile for road riding.
