COST is the main factor. I dun have $2600 to spend on a 28-70 from Nikon, thus I got the Tamron 28-75 ($600 or less) as an alternative. Much much cheaper and I dun require the top end quality from Nikon as yet. Do note that 3rd party lenses often have different colour casts that may differ from the originals. And also some of the 3rd parties have problems with distortion, etc but if you can forgo these problems, go ahead.
Generally, I'm in agreement with this, but not fully.
Cost is only 1 of the factors. However, the build is another important factor which many don't realise and I will explain why.
Many of Canon's cheaper lenses and a few L lenses have a lesser robust, or more vulnerable, build. As you probably know, lenses are tubular (comprising of tubes/barrels). After some use or impact, the inner barrel won't feel as tight/firm, and in some cases, wobble and droop to the pull of gravity. This is when the lens resolution becomes compromised. This problem adds to the mediocre optical performance of the lens and makes the lens even more crappy. One good example is the cheapie 50mm f/1.8. (The Nikon 50mm f/1.8 is built far better than the Canon.) Some others are 18-55mm, 17-85mm, 28-135mm, 24-70mm L, etc.
In comparison, some third party lenses are much better built and hence more durable. It appears that the Tamron 28-75 and Sigma 24-70 are less vulnerable to wobbling/drooping inner barrels than the equivalent from Canon. This is a serious plus point.
As for lens resolution/performance, by test results, Tamron appears to be a bit better than Sigma. In fact, they have been making very good lenses since back in the days of the Adaptall mount for MF lenses. (They were 1 of the most expensive third party lens makers.) Tamron's 17-35mm f/2.8-4 has been independently tested to be on par with, or as good as, Canon's equivalent in terms of optical performance although it's very understandable that many Canon 17-40mm L uses will aggressively, violently, religiously, vehemently, murderously, dismiss it as hogwash but it is not.
Another good Tamron lens is the 28-75mm f/2.8. Although I haven't had any personal experience comparing it with the Canon equivalent, this lens is obviously sharp, you wouldn't be quick to classify it in the category of a consumer-budget lens like the kit lenses 28-80mm or 28-90mm coupled with film bodies. This Tamron is sharp, and for it's less vulnerable to inner barrel wobbling/drooping and amazing compact design, is likely to outlast the Canon cheapie zooms. It's price makes it a no-brainer why you would buy this. If there was someone open-minded and neutral to test this with the equivalent L, I'm not surprised that if it is a fair fight between them.
All lenses, L, ED, G, or whatever manufacturers call it from A-Z, have it. All wide angles have barrel distortion; the wider it is, generally the more distortion. All telephotos have pincushion distortion. This is something almost impossible to fully escape from and can only be minimised. Hence, the problem is not only confined to 3rd party. Never believe that the L or ED is the perfect lens. If you have a resolution target for test lenses, you can see that even L lenses have pronounced chromatic abberation (CA) at wide angles. Pronounced enough to wonder why you blew all that money away when the 3rd party isn't any worse.
Do keep in mind that when buying original lenses, the hefty price tag has probably taken into account other things such as, the larger production, and longer continuity, of spare parts. With 3rd party lenses, some could be considered to be like single-use. Once banged up, it's too cheap and uneconomical to repair, and they don't have/keep parts for it. If you're a Tamron and Sigma owner, you probably know this.