The are forms of interference that will get through no matter HOW short your cables are. It is a limitation of unbalanced lines.
Shorter cables gives mostly the benefit of less high frequency loss, but there are types of interference that don't much care if your cable is 1' or 10".
A balanced circuit eliminates most interference problems with some nifty tricks done in electronics.
You can read all you want to read about balanced/unbalanced lines here:
http://www.dself.dsl.pipex.com/ampins/balanced/balanced.htm
Another problem associated with using insert jacks is that the "normalling" on them start to wear out. I am almost willing to be that 9 out of 10 people who have a channel "go out" on their console when they have been using their inserts jacks a lot simply have a problem with the jack completing the "normal" when no jack is inserted. I have seen it on MANY consoles from my years of live use.
What happens is that the "send" and "return" and physically linked together on the jack until you plug in a cable, then the physical connection between them is broken. After enough insertion on the insert jack, the metal fatigues, and quits making a good "normalling" connection when nothing is plugged into the insert jack. It is not a "rare" problem, rather, a problem that happens on many mixers after a lot of use on the insert jacks. It is the number one "fix" we do on the live mixers at the sound company I work for.
So, while using the inserts will work, again, it is not as reliable as making full connections.
I have also seen where some 1/4" connector/jack combo's don't "fit" so well. One connector makes a good connection, and another brand doesn't. Sound funny, but again, a problem I have seen many times. The fix is simply a different 1/4" connector (on the cable). But, this can be annoying, and if you didn't know that it COULD be a problem, you would never suspect that it is.
Again, tapping an insert works in a pinch. It just comes with it's own share of potential problems. If you aren't having any of those problems, then you aren't having them.