You have to look at photography as a package deal. Each and every part of the package is important. Technique first and foremost is the biggest part of the package. From there, you have to ask what camera body is going to suit my needs. Some of course do faster burst for sports and wildlife, some slow for landscape. As for the glass, it is no different than eye glasses, some have coatings for glare, as pointed out specs are much closer on expensive glass, and optical formula is important. That is where the design of the lens can give you sharp images throughout the zoom range.
Where you will see the biggest difference in a more expensive lens is contrast and detail sharpness. In a nutshell, light focuses at different points, red, yellow and blue all are sharp at tiny different spots. More expensive lenses have super high quality multicoating that will aid to focus all the colors at the same point...your sensor. Another difference is the number of imperfections in the glass elements. The slower the glass is cool, the less imperfections. It is more costly to allow the glass to cool and the quality standards are much higher and will therefore demand a higher price at market.
If you are happy with what you are using and the results are meeting your standards, then don't get more expensive glass. If there are things your lenses don't do for you or when you look at the details, you feel you need better glass then get it. That is what drew me to Tamron lenses in the 70s. I couldn't afford the "expensive" glass and I needed to do some macro work. I gave the 90mm a go and have been using them ever since. I have 2 types of lenses in my bag, Tamron zooms and a couple of Zeiss prime lenses. I use the Zeiss primes less than 1% of the time. I hope this helped a little and didn't add to any confusion.
