Even though the wheezy packages may be directly installable on squeeze
today, making the squeeze-backports version available is a useful signal
to administrators that a package has been successfully built against the
older release and is believed to work.
It's also probably not a good habit to get into either even if it happens to work in isolated cases. Judging from conversations on #debian mixing distributions is a very common way of breaking your system. ;o) Using squeeze-backports is much safer.
Especially now we have multiarch in Wheezy so libraries won't be installed where you'd expect under Squeeze.