I guess this topic will go around and around for as long as it takes Canon to change their policy and recognise that there are professional photographers out there that use crop sensor cameras, namely the Eos 7D, which is manufactured to "pro" specs anyhow. Canon promote the EF-S 15-85 f3.5-5.6 IS and the EF-S 17-55 f2.8 IS as the "standard" zoom lenses for the 7D. They do not manufacture an EF-S lens with the "L" designation. It is a misconception that L lenses are optically superior to non-L lenses. They are not, but they definitely are mechanically superior. An L lens is designed to be as bullet proof as a lens can be and still carry a reasonable price tag.
The EF 24-105 f4 L IS USM is now quite an old design and in my experience, the image quality is not as good as that of the EF-S 15-85 f3.5-5.6 IS, and is particularly poor (relatively speaking) between 80 and 105mm. The EF 24-70 f4 L IS USM has very good image quality and has the same bullet proof build quality, but has s fall-off in edge resolution when used wide open at 50mm. The 24-105 has been tainted with the "high-end kit lens" label because it is offered with the 5D III and the 6D bodies. Be assured that it is in no way inferior to any other Canon "L" lens and is built to the same standards as all Canon professional lenses. I owned both the EF 24-105 L and the EF-S 17-55 f2.8. I have sold the 24-105 and replaced it with a EF-S 15-85 because I will be travelling extensively in Asia later in the year, using a 7D, and I want the wide angle capability of the 15-85. The weight of the 24-105 is also PITA when travelling light, as is the 17-55 f2.8, which also suffers from extreme axial chromatic abberation at all apertures, and unless fitted with a UV or protector filter, will injest dust and moisture like a vacuum cleaner. Why don't I use an EF 17-40 f4 L USM, you ask? Because I want and need IS capability.
So, my advice is, if you want a walk-about lens for use on a crop frame body that offers high image quality and is fairly light weight, get the EF-S 15-85 f3.5-5.6 IS USM, unless you are a movie buff and need a constant aperture across the entire zoom range, in which case you have no choice but to opt for the EF-S 17-55 f2.8 IS USM or one of the "L" lenses.