Yes you should consider third-party lenses.
For years I've enjoyed lenses from Tamron, Sigma, and Tokina. Zeiss is another good brand. All these manufacturers make superb lenses as well as duds. Do your research before buying one of these lenses to know what you're getting. If you don't want to do your research then buying a Canon L is pretty much a guarantee of quality.
As for sample variance. All manufacturers including Canon, Nikon, Tamron and Sigma make duds. It's probably true that Tamron and Sigma are more prone to manufacturing defects like decentering. That said, buying an L lens is no guarantee. My friend bought a badly decentered 70-200/4. Canon eventually fixed it, but it took six months. Read the photozone reviews about 24-70/2.8L and 24-104/4L for more stories of bad copies of L lenses. If you're concerned about getting a bad sample, buy from a bricks+mortar shop who will stand behind their products with exchanges and refunds.
I think it's a real shame - IMO Canon should test every individual 70-300L before putting it on the shelf. This way "L" would actually be a guarantee of quality. "L" would actually mean something.
It is said that 3rd party manufacturers are no insurance for future compatibility. Perhaps this is true. However Canon has changed their lens mount too, rendering old lenses useless. Today, a lot of FD glass sells for pennies to the dollar because of this. When Sigma lenses started misbehaving years ago, Sigma offered free chip upgrades.
So, let me make an analogy. Suppose you ask what's good for dinner and someone suggests filet mignon (Canon L). It's certainly a good suggestion, one you can't go wrong with, but there are other cuts of beef out there (Canon non-L) as well as pork and chicken (Tamron and Sigma).
I love my Tamron 17-50/2.8, Tamron 90/2.8, and Sigma 10-20 dearly. These lenses have performed for me year after year. They don't owe me anything, and I don't care about their resale value.