The 16-35mm f/2.8 may make sense when you go FF, and don't have the 17-55mm, but for now, already having a 17-55 f/2.8, I don't see the point. It's also a super wide angle for full frame, so you'll be in for a surprise when you upgrade cameras. If you want to go super wide angle on a crop body, you need to look at the 10-22mm, which will frame exactly the same as a 16-35mm on full frame.

With the 16-35mm get 1mm that wasn't already covered, no aperture difference, no new shooting possibilities until you upgrade to full frame. You're not even saving yourself a lens swap (For example, having a 10-22mm and a 24-70mm can get annoying if you tend to shoot 17-30mm. You'd have to keep bouncing back and forth between lenses. In this case, adding an 17-55mm can be justified, despite not really covering any new focal lengths, as it saves a ton of annoying lens swaps.)

So, no, for now I don't see the 16-35mm as having ANY value to you, while the 35mm opens up low-light shooting.

Or, instead of the 35mm, save the money and buy a flash. They're cheaper, and will provide you plenty of light to shoot indoors with the 17-55 you've already got. Which works best for you depends on YOUR preferences and situation. Do you think your subjects will stick around while you find the flash, assuming it doesn't stay on the camera at all times (the same argument can be made for the lens!)? Do you prefer the look of natural light? Are you willing to accept the thin DOF you'll be forced to have to get a decent shutter speed without a flash? Are you willing to work within the limitation of a fixed focal length?