your 17-55 example of selling at a rate below other 3rd parties is slightly flawed. While everyone is stating canon lens hold their values, it needs a qualifier and that's canon EF mount lens. I've seen ef-s lens lose quiet a bit more value than EF lens.
May people here said "Canon lenses have better resale value". None of them said "Canon EF Mount lenses have better resale value". By the way, you know the Sigma 30mm F/1.4 is EF-S mount, right?
Let me make it clear.
1. Canon really makes a lot of good lenses. That's why I have many Canon lenses.
2. A third party lens can hold it value well if the lens is really good. I don't know how good the Tamron 24-70mm is yet because I haven't see some helpful reviews.
3. Canon lenses DO NOT always have good resale value. Please search Canon EF
24-105mm L on CL. The resale value is low as well.
Too many people just post here without proof. Some of the posts really bias on Canon. They just against anything not good for Canon. That makes me feel they are posted by CANON EMPLOYEES.