1. It cheapens the whole process of grading. You simply CANNOT evaluate a book from a scan. Why would the CGC even try?
2. It insults my own ability to evaluate a book before I sell or buy it. If I can't reasonably tell if a book falls somewhere within the VF to Gem Mint range I should probably get my glasses checked or find a new hobby.
3. Bluntly - it's a ripoff that makes the CGC come across as money-hungry, opportunistic capitalists. I see no value in the service unless you're a hopeless newb with money to burn.
4. If I buy a book and disagree, I can usually return it - hopefully through the seller's policy, but surely through eBay's policy. So why do I need the CGC's service?
5. As mentioned previously, what if a centerfold is missing? Interior piece torn? Restorations that wouldn't show in a scan? Married covers? The new service can't detect these flaws. Again, you can't grade by looking at the scans. What if I buy an "online evaluated" book, and I run into these problems? Will CGC refund the money for evaluating the scan?
6. The grading ranges are far too broad to be useful at ANY grade point. Only the very worst graders and the most clueless people in the hobby would be off more than 2 points.
There is already a perception that 3rd party grading services are rip-off scams...and this perpetuates that notion. It's just not a good look in my opinion. I see it as the equivalent of McDonalds charging you extra for ice cubes and straws.