I tend to think I'll need everything so I often take everything and the I get a full and heavy backpack. I want to avoid this trip.
& now you start to understand why I haven't bought a 70-200 f/4L IS or 70-300L IS yet. I've been getting by for half a decade with my longtime friends the 135L and 1.4x TC, & now that I've got a 6D that produces pretty clean shots at ISO fifty billion & has lower pixel density than my beloved 40D (& thus is less sensitive to camera shake) the need for a long IS telephoto just went down considerably. Maybe I'll upgrade the TC to the III version instead or perhaps get a 2x to compensate for the loss of "reach".
But that's one less big lens I have to figure out how to carry while still being able to cram the bag under an airplane seat and not breaking my back when I drag it around for hours.
Because I always want the 135L with me :-).