Ah yes, Mark talking about recording on-track versus on a dyno. There are two very big detractions from this technique: cost and complication. Let me talk about them for a bit.
Cost. A dyno shop typically charges $100-$150 per hour for dyno time. When I did a dyno recording of my own car, I went exceptionally slow and did it in 4 hours (since I was still learning what to do.) I'm confident I could do it again in 2, maybe less. Recording on a racetrack is a different story. You have to rent the track facility, which is usually a per-day charge, and after crew and insurance and a driver, you're usually looking at $10,000 for most local tracks. Oh, and you better hope it doesn't rain, or that's a day wasted. The amount of time spent recording per-car goes up quite a bit too, which is the second point:
Complication. As Mark alludes to, recording on a dyno is much more controlled. First off, you can physically adjust microphones while the car is running so you can hear your changes in real-time. Second, the dyno is infinitely better at controlling the dynamics of the car itself versus a driver on the track. Recording on the track means you place your mics and recorder within the car, hit record, do a lap, bring the car back in, listen back to the recording, make adjustments, and repeat until perfect. As you can imagine, this adds quite a bit of time to the recording process. And then once your mics are in the best spots, the driver's performance needs to be spot on to get the material you need. I haven't done an on-track recording of a car but I imagine it's closer to the 3-4 hour mark per car just in recording time. Once those recordings come home, they then need to be edited, too - and while neither one is particularly quick to do, the on-track recordings will take much more precise cutting and stitching to get into a useable asset if using a loop-based system. Codemasters use a different type of audio system called "Granular synthesis"* which is better-suited to how they can record their tracks so it's probably not at 1:1 comparison on which is ultimately easier to edit, but I would take an educated guess that it takes the CM team somewhere around 15-20 hours of work to get a recording into a useable asset, whereas I'd guess T10 is spending 4-6. Would love to know more about that though.
And then there is the difference in sounds that are captureable, too. In a dyno facility, you can do things like take body panels off and stick microphones where they sound the most natural, like within the intake system or just outside where the hood would be when it is shut. However, on an on-track car, you can't really remove parts of the car safely to get mics where you really want them since the mic has to be attached to the car (and usually inside of the body panels to escape the wind). Also, on a dyno you can put mics further away from the car - think about a car sound in your head, how far away from the car are you when you listen to it? Your average "listening distance" to what you think a car sounds like is probably something like 6-8' away, which is the difference in height between the exhaust pipe and your ears, plus the linear distance you're probably standing away from the car at a car show or similar, right? If you have a car, try this: start it up, let it idle, and stand a few feet away from the car, listening. Now, take your head and put it next to your tail pipe with your ear against the bumper. It's a little different, eh? With on-track recordings you are limited in mic placement by the car itself - you can't really put mics much further outside of the confines of the car. On a dyno, though, you can put one a few feet up and back if you want, because the car isn't moving.
Granted, the further away from the car the more you have to be concerned about reverberation within the dyno room. And dynos all tend to have some roller noise. But on-track recordings are not without flaws too - you have reflections from the road, tire and debris noise, and ambient noises to be worried about too, so in my eyes those cancel each other out in terms of complexity.
There is also one other issue - getting access to cars is already hard enough, and getting to record them is probably quite difficult. However, imagine you owned an ultra-rare Lamborghini, let's say. Which is going to be easier to convince you of: letting an audio team run the car on a stationary, indoors dyno for 2 hours, or letting that team flog it on a track for 4-6?
Doing on-track recordings for a studio like Codemasters makes a lot of sense. F1 games really only need one recording set (with some DSP changes to make the cars sound a bit different), so it makes sense to spend a ton of money getting one really-kickass recording and dumping tons of work into getting it sounding just so in-game. However if you try to extrapolate this out to even a 200 car list game (nevermind something like GT's 1200) it just isn't an economically sane choice. One option that has been thrown into the gt6 "better sounds please" thread was just having a typified engine sound - and indeed CM has done that in the Dirt series, with some cars in D3 having the same recordings (ie having a "subaru" sound and a "turbo 4" sound and a "truck v8" sound). I think this creates a great way to have high-quality sounds across a wide range of content - but at the sacrifice of variety which I'm not convinced the typical racing game fan is willing to give up (perhaps I'm wrong though). CM did the thing that makes their game as awesome as it can be - I just don't think it's valid to suggest that their solution is the best solution for every game, and I think Mark would agree with me on that.
*the granular synthesis method is also interesting because it takes up less memory (essentially you're saving a 10-second stereo audio file versus a multitude of individual ~1 second loops) but requires more CPU to process what should be playing (whereas the loop system takes very little CPU), which I think is why CM's games sound quite good on ps3/360-gen consoles, where RAM was a significant factor. What they do when they move to next-gen should be really fascinating and I can't wait to hear it.
TL;DR: on-track works great for CM but I'm not convinced would work well for T10/PD