Why do system users have valid shells

[Tobias Reckhard <jester71@gmx.net>]

Hi

We recently noticed that a stock woody install produces an /etc/passwd 
in which most, if not all, system users have a valid shell entry of 
/bin/sh. They're all unable to login due to having no valid password, 
but best UNIX security practice typically involves giving accounts that 
don't need to be able to login a shell of /bin/false or /bin/true. Other 
distros (at least some of them) appear to follow suit.

Is there a reason why Debian chooses to specify /bin/sh for system 
accounts? Do we risk breaking anything if we perform an 
s/\/bin\/sh$/\/bin\/false/ ?

Cheers,
Tobias