jQuery would never have come into popularity if not for oldIE and its often dramatic variances from competitors: If you are willing to forsake that old browser, there is little added but a layer of inefficiency between you and web standards. We have become so accustomed to jQuery that it becomes the default, many unaware of the dramatic improvements in all major browsers since.
As to the cost, assuming that everyone is pulling it from the same CDN (and there aren't the often considerable times simply to check an etag, which is the case with the overwhelming majority of jQuery hosts), an iPhone 4S takes about 80-100ms to simply parse the jQuery file (and given the plateauing of the integer core on on the An chips since, it is a reasonable assumption that every Apple variant since is similar). On each and every page render. A tenth of a second is a long time (and a lot of battery) for what is often of minimal value.
As to the cost, assuming that everyone is pulling it from the same CDN (and there aren't the often considerable times simply to check an etag, which is the case with the overwhelming majority of jQuery hosts), an iPhone 4S takes about 80-100ms to simply parse the jQuery file (and given the plateauing of the integer core on on the An chips since, it is a reasonable assumption that every Apple variant since is similar). On each and every page render. A tenth of a second is a long time (and a lot of battery) for what is often of minimal value.