I've been debating adding a f/2.8 standard zoom to my kit to use for lower light and indoor family and event photography. I currently have the following lenses that I use on my Canon 6D:
24-105mm 4.0 L
35mm 1.4 L
135mm 2.0 L
70-200mm 2.8 II L
I normally use my 35/50/85/135 primes for low light photography and they do a great job, but I find myself often wishing I had a fast standard zoom to avoid frequent lens changes or missing shots with the kids who are moving faster than I do.
I use the 24-105 for most outdoor and landscape shots. Its a good lens, but after extensive use this winter, I'm a little disappointed with its IQ, distortion at the wide end and I don't think colors are rendered quite as well as my other L lenses. So, if I buy a 24-70, It will probably become my primary landscape/outdoor lens as well and I may sell the 24-105 at some point.
Budget isn't a major consideration. I have the money available for the Tamron lens now and can have the money for the Canon II in a month or two.