Eeek! Hold on a second... :)
You can't rule out lines of descent with an autosomal test because a person isn't a match to you. (Well, unless you're talking very close relatives.)
That cousin may have inherited a completely different segment from your common ancestor. That's why it's good to test as many people as possible. Your sister may match a 4th cousin that isn't on your list, and vice versa.
Also, you may have a segment in common with someone, but it's just below the reporting threshold and won't show up in your list of matches. I have some (if I remember correctly) 6.9 cM matches on GEDMatch with a clear paper trail connection who don't show up in Relative Finder on 23andMe (where the threshold is 7 cM, I think).
What I would like to see is discrepancies noted when someone takes or enters their yDNA or mtDNA results. I put in my mtDNA results and Ancestry gave me the option to tag each member of my direct maternal line with the results. But what if someone else with a matrilineal descent from our ggggg-gm has a different haplogroup? I would like to have that pop up as a hint. It could be very useful in determining the existence of other wives or in perhaps assigning the wrong woman as a wife at some point.
(Ditto for the yDNA test.)
I do think it would be good to see DNA matches integrated into the family tree database, but it might be a headache to implement, since it would mean generating a custom view of the other person's tree. It's also something that needs to happen carefully; we don't want people to get sloppy and say, "Hey, this person is a DNA match and they're related to my ggggg-gf. This MUST be our DNA connection!"
But yes, I would like for my DNA matches to be flagged and linked to our match page when I'm looking at trees, especially since we're soon going to have so many matches that getting to each one in a timely manner will be difficult. I would suggest this via the feedback button, but I don't want to distract The Powers That Be from getting us some segment matching first. ;)