I thoroughly agree with the majority of your points here. The biggest exception is the statement, "the coin being slabbed is surely not a counterfiet". Well, the vast majority of the time that is quite accurate, but I have seen a few and heard about many more coins that were slabbed and turned out to be fakes.
Also, if a slabbed coin gets stolen, it is a rather simple matter to crack out the coin. Then there is a raw coin without any serial numbers to trace.
I have purchased both slabbed and raw coins for my collection. The vast majority I bought raw because, in most cases anyway, I trust my ability to grade and spot problems such as cleaning. When I buy a slabbed coin, I look at the coin, not the plastic tomb or its label because I don't give a rat's rear-end what the TPG thinks about the grade. I listened to enough collectors and dealers tell me to "buy the coin, not the slab" - so that's what I do.
The thing that irritates me most about TPG's is the adverse way they have affected the market, particularly with MS coins. I guess I don't understand why an MS-65 coin is worth $1,000 but the same coin reslabbed with an MS-66 grade is worth $7,500. And this happens frequently. People will buy a nice 64 and crack it out, submit to another company (sometimes even the same TPG !!!) and it comes back at 65. I know people who do this all the time. It's fine for these dealers/speculators/investors to make money - it's just beyond my comprehension why anyone would pay such huge differences in price because of a one point difference in grade. As you said, grading is highly subjective, so why do most collectors assume that the TPG is more accurate than anyone else? Besides, can you really spot the difference between a 65 and a 66? Or a 68 and a 69? With that, I will end my little rant.