I'm inclined to agree with Vlad, in that I suspect we hear frequent cases of strings slipping out of 12-hole tie blocks because players believe they are more secure, or simply don't know how to tie them correctly. I've been using 12-hole tie blocks for about 3 years now I think, and tying them with the method shown on my website, and I've never had any problem with slippage. To be fair, I do always use nylon strings, and carbons do seem to be more prone to slipping. This is why I do recommend melting a ball on the end of the string. I've tried pulling/breaking the melted end off, and it's not easy, and don't forget it doesn't need to hold the full tension of the string, just provide a bit of extra resistance to slippage.
James Lister, luthier, Sheffield UK