On the other hand, have you ever seen a large, heavy L lens mounted on a rebel. The balance seems like it would be quite awkward with a 24-70 L or a 100-400 L.
I tried a 100-400l on a 300d/rebel, and as long as you just carry it
by the lens it's not that awkward (not very convenient to point upwards though)... The whole lens+camera set has just the weight of a dead donkey (french proverbial).
Even for lenses, I prefer the lighter 55-250IS to the heavier L offerings, for the same weight (and price) reasons.
I'm often amused that this 55-250 gives me ca. 100-400mm equivalent,
with same apertures - and better stabilisation from what I've heard, just 4 times lighter and 6 times cheaper, and the sharpness difference with the 100-400L I tried is not that significant on my old rebel. I don't wanna go full frame!
Yes, sometimes I wish I had the real-USM AF on my plastic toy...