The optimal clutch power is attained after 8 to 10 couplings(search for "clutch"); that is, before that, the bricks will sometimes cling a little too much on each other.

However, I don't think anyone ever actually stated when clutch power would start to deteriorate. I'd tend to assume that once the optimal clutch is reached, there is just enough plastic at the coupling to make it work, but not enough that it would wear of.

Of course, as you said, I suppose there is still a little that wears off, but I'd wager it isn't that much, and it's probably negligible with regards to the manufacturing tolerance of the bricks. In other words, you're more likely to find two bricks which come loose too easily because they're at the limit of the tolerance, each at a different extreme; than to take two bricks with normal clutch power and try to wear them off so that they too come loose easily.

Then again, I don't think it's ever been extensively tested. We'd need two bricks, and a robot that assembles/disassemble them a great many times. (Unless you want to do it manually)