Why packages in testing/unstable require the newer shared libraries ?

[Boris Toloknov]

Hi All,
I have debian stable (etch). Sometimes I want to install the newest 
versions of some packages from testing/unstable. Typically I can only 
install some package with the newer version of the shared libraries it 
depends on. For example dash (0.5.3-7) from etch depends on libc6 
<http://packages.debian.org/etch/libc6> (>= 2.3.5-1) while dash 
(0.5.4-12) from lenny depends on libc6 
<http://packages.debian.org/lenny/libc6> (>= 2.7-1) and debian stable 
has libc6 (2.3.6.ds1-13etch7) .

Will my system be broken after upgrading libc6* along with locales, 
tzdata and binutils from testing(lenny) repository ? If not then it 
means that other programs ( for example bash ) are able to work with 
libc6 <http://packages.debian.org/lenny/libc6> (2.7-*) instead of libc6 
<http://packages.debian.org/etch/libc6> (2.3.[56]*). On other hand I 
don't think that dash (0.5.4-12) from lenny requires something that is 
absent in libc6 <http://packages.debian.org/etch/libc6> (2.3.[56]*). Why 
in that case dash and most/all of the program packages from testing 
require the newer libraries from testing instead of stable ?

Typically I want to upgrade just some package like gcompris from 
testing. What options do I have ?
1) Upgrade the package along with a huge set of libraries it depends on. 
But I don't want to make all my system testing/unstable.
2) I can get the source of the newest gcompris, lib*-dev packages, 
compile it and remove the source and lib*-dev. But it takes a lot of 
time. Moreover I can do it but my wife can't and she doesn't understand 
why it takes that much time to just install a newer version of some program.
3) Wait for a couple of years for the next stable release of debian. 
It's too long.

I read 
http://tldp.org/HOWTO/Program-Library-HOWTO/shared-libraries.html, 
http://www.debian.org/doc/debian-policy/ch-sharedlibs.html and some 
other articles however I don't understand: Why packages in 
testing/unstable require the newer shared libraries ?

Thanks,
Boris