on a system with many users (say at a university) where you don't know or trust the users, you still need to be able to impose these sorts of restrictions effectively.
Just hoping for best seems to work in practice, at least in my university (cs.helsinki.fi)
nikki@melkki:~$ ulimit -a
core file size (blocks, -c) 0
data seg size (kbytes, -d) unlimited
file size (blocks, -f) unlimited
max locked memory (kbytes, -l) 32
max memory size (kbytes, -m) unlimited
open files (-n) 1024
pipe size (512 bytes, -p) 8
stack size (kbytes, -s) 10240
cpu time (seconds, -t) unlimited
max user processes (-u) 32755
virtual memory (kbytes, -v) unlimited
I'd also like to mention the network has 3 terabytes of diskspace without quotas. As long as everyone acts nice, things work fine