Once someone has bet sports long enough to think they're sharp, they start accumulating these "Rules to bet by" which are usually just a fallacy. One of them is there is no value in laying big chalk.
The simple answer is any bet at any given price can have value, be it 50/1 or 1/50.
I won't give out many plays but to give an example for this thread I just bet Kaymer NOT top 5 at -400.
According to the market this has at least an 85% chance of winning which is well beyond the 80% needed.
The market is still not very mature with more than a day and a half left, but there's enough of an overlay to warrant a bet right now.
The thing to remember is it may lose and that didn't make it a poor bet because you can't judge betting decisions in a vacuum. Just like someone laying Justin Rose to win at -800 would be a great bet even with him being the hottest golfer on the planet and even if he does win.
I agree that if you don't know how to either price something or read the market, you shouldn't be laying huge chalk because you'll be paying a premium price for your mistake. But once you understand these things, you shouldn't discount any bet