Earlier revisions had some bugs (errata) but they still work fine and the problems can be worked around. For instance the MMULibs software checks the revision of the chip when the computer boots and disables some problematic features. Doing that has a slight hit on performance but it's probably not noticeable.
The main reason why chips with revision 6 are so sought after is instead their ability to be overclocked. Motorola and Freescale never released any version of the 68060 rated for more than 75 MHz, but at least the full version of the chip with both MMU and FPU at that speed are really uncommon so the vast majority are only rated for 50 MHz. But for instance I run a 50 MHz revision 6 chip at 66 MHz without any cooling in an Amiga 1200. With a proper heat sink and fan it could easily do 80 MHz and if you're lucky with the silicon lottery 90-100 MHz isn't uncommon.
That's quite a lot faster than the earlier chips. From what I've heard, the first revision 060 can often be overclocked to around 60 MHz and revisions in between might handle around 75 MHz. And that's only with a decent heat sink and fan.
The sixth revision was also produced with a smaller manufacturing process so it runs cooler. That's nice in a computer without fans like the Amiga 500 or 1200.
This combined with the fact that several new accelerators for common Amiga models was announced made the rev 6 really rare. The creator of one of the projects abandoned it before it was released and the Warp 1260 has been delayed but both of those projects required the user to supply the CPU, so users bought the chips last year to prepare. That made the price go up and the market was flooded with fake chips. The fakes are often earlier revisions, sometimes without FPU or MMU, that has been rebranded.
Like with anything retro computing related nobody really needs a rev 6 but if you're going to accelerate your decades old computer, why not go as far as you can?
