Have to? No. There's no have to. If you look to both sides you will not see a gun pointed at your melon. If you do, you've got worse problems than a truncated post.
Need to? No one would need to if the person would simply delete the rest of the post and quote only the part they were really interested in commenting on. You're supposedly all for truncation but only after more characters are added. But if someone does this at the very last paragraph of a 24 inch column will it make any difference? It is still going to be cropped off where the original poster could simply crop the top off and quote what he found interesting to respond to.
In other words, perhaps more posters will clean up the quoted posts and make this a moot point so that their point gets across. We can dream.
Regardless, once you click the full view the quoted post is fully displayed. Unless fingernails are breaking by that one click I really just don't see what the big damned deal is.
I'm testing to see if it is possible to post sub rosa messages and images.
The image is of kicker Matt Bryant.