I would agree. You're unlikely to miss the 51-69mm gap, though if the 28-135 would be of interest to you otherwise as a lens that covers a wide range of focal lengths that you use a lot it could be useful as a general purpose lens you keep on the camera all the time. When included in a "Kit" it can often work out quite inexpensive.
For example Amazon have the
Canon EOS 7D for $1599 and the
Canon EOS 7D + 28-135 kit for $1799, making the lens only $200 (it the
Canon EF 28-135/3.5-5.6 IS USM sells for $400). So if you ever intend to get a 28-135, then it makes sense to buy it as part of a kit with the 7D.
If you just wanted to cover the "gap" the
Canon EF-S 60/2.8 macro USM might be a good choice since it is an excellent macro lens as well as a general purpose 60mm prime and so doesn't really duplicate anything you have now.