Andrew_WOT
Leading Member
You are absolutely right, spending same $1000 plus on cheaply build sucking dust plastic EF-S with prone to failures IS and useless on FF would make them feel much better. 
Focal range wise they are apples and oranges. If you want to cover 17mm range, get 17-55 or if you plan for FF and prefer better built stuff, 17-40 or 16-35 II, but neither is a substitution for 24-70, rather a complement. They all are missing important for portrait work 50-70 focal range and all image stabilization in the world will not make up for that.
From "Canon Portrait Lens Recommendations"
http://www.the-digital-picture.com/Canon-Lenses/Canon-Portrait-Lens.aspx
Now with 24-70 you'll get 38-112, 27mm worth of coverage, that is you have 85-112 versus 85-88 on 17-55, that's translates into much more flexibility and ability to get a nice close up w/o getting into face of you subject. I personally rarely use anything outside that range on 24-70, just having 17-55 alone would make absolutely no sense for me.
Just go after focal length you personally need the most now and fill the gaps later.
Focal range wise they are apples and oranges. If you want to cover 17mm range, get 17-55 or if you plan for FF and prefer better built stuff, 17-40 or 16-35 II, but neither is a substitution for 24-70, rather a complement. They all are missing important for portrait work 50-70 focal range and all image stabilization in the world will not make up for that.
From "Canon Portrait Lens Recommendations"
http://www.the-digital-picture.com/Canon-Lenses/Canon-Portrait-Lens.aspx
Let's put some math to work, 17-55 on crop translates to 27-88, you have 3mm of ideal portrait range covered.Conventional teaching is that the 85-135mm focal length range is ideal for portrait photography (field of view crop factor included).
Now with 24-70 you'll get 38-112, 27mm worth of coverage, that is you have 85-112 versus 85-88 on 17-55, that's translates into much more flexibility and ability to get a nice close up w/o getting into face of you subject. I personally rarely use anything outside that range on 24-70, just having 17-55 alone would make absolutely no sense for me.
Just go after focal length you personally need the most now and fill the gaps later.
Yes it's a valid reason long as you know that's what you are spending your money for. Spending $1000 to buy an L lens is as jusitifiable as for people who spend $1000 to buy a disigner bag for that feel good factor. On the other hand it's weird that people always try to invent those funny reasons (IS is not useful, IQ is not that important, wide end is not necessary, it's weather sealed for a non-wheather sealed camera, it fits full frame camera that I don't have.....) to defend that his/her money was not spent on a better lens for taking better photographs.The only reason someone with an entry level crop body will pick a 24-70 is because you are blinded by the "L" and want to own one. But I think thats a valid reason though, L lenses are just nice to look at while the 17-55 looks like a giant kit lens.