I believe in keeping things simple, so for the last few years I’ve been carrying only two lenses – a Canon 24-105 L, and a Canon 100-400 L. Between the two, I can cover the range from 24 mm to 400 mm without fumbling around with a lot of lens changes. I realize zoom lenses with long ranges come with an image quality trade-off, but the convenience of keeping my workflow simple in the field is worth it to me. I would rather spend my valuable field time seeing, reacting, and shooting, rather than having to make a bunch of lens changes. And, in fact, I would guess that well over 90% of my images are made with the 24-105 L (and probably half of those at either 24 mm or 35 mm), so my workflow in the field really is straightforward.
Still, I do think the 100-400 is worth keeping around. There are some situations where the reach really comes in handy. This image, for example, likely would not have been possible with the 24-105. It was made with the 100-400, at 400 mm. This enabled me to: 1) shoot from across a busy highway; 2) isolate this pattern from its surroundings; and 3) create a very flattened perspective (due to the telephoto effect), accentuating the graphic aspects of the composition.