a 10% or 51% attack is possible
Ok, let's clarify one thing.
This claim that getting a miner having 10% of the hashing capacity to be able to reject transactions is referring only to being able to get six blocks in a row. This is no different than coin flip trials ... yes eventually you will get six tails in a row. Eventually you will get sixteen tails in a row. Eventually you will get six thousand tails in a row (though chances are none of us would live long enough to see it happen).
Now, because there is a financial penalty for each failed attempt (as you are using expensive hardware and consuming elecricity, but not earning any bitcoins on the failed attempts) then there is less and less chances that you will make this attempt. The logic is, if you have 10% of mining capacity you might as well mine and earn 10% of the bitcoins issued.
Next, let's say that someone does take their million dollars worth of ASICs and do this to prove a point. OK, so they try for an entire month and do actually get six blocks in a row. They won't get seven though. Not with only 10%. So at best, this attack will cause double spending to occur for transactions in the first block for each side of the blockchain fork. But the attacker is going for an entire month of trials, not knowing which block will be the one that the attacker follows by mining five more in succession.
So sure, you might get six blocks in a row. But you aren't going to be causing much financial harm as a result. Every exchange has AML policies requiring identity for any significant amounts of funds (e.g., amounts over $1K USD per day withdrawal). You might be able to get a withdrawal out. It won't be a significant amount though.
So yes, an evil miner with 51% attack with 51% of the hashing power would be terrible for bitcoin.
But an evil miner with 10% of the hashing power and a good string of luck would simply be providing a learning lesson (e.g., for an exchange/E-Wallet provider to start requiring more than six confirmations for larger transfer amounts ... something one might realize might have already been put in-place. if you have your ear to the ground.)