use RPATH but not RUNPATH?

When you ship a binary, it's good to provide means for the users to accommodate the binary to the specifics of their own system, among other things, adjusting library search paths.

A user can generally tweak LD_LIBRARY_PATH and /etc/ld.so.conf, both of which are with lower precedence than DT_RPATH, i.e. you can't override what is hardcoded in the binary, whereas if you use DT_RUNPATH instead, a user can override it with LD_LIBRARY_PATH.

(FWIW, I think ld.so.conf should also take precedence over DT_RUNPATH, but, anyway, at least we've got LD_LIBRARY_PATH).

Also, I strongly disagree with the suggestion above to use DT_RPATH. IMO, its best to use nether DT_RPATH not DT_RUNPATH in shipped binaries.

unless

you ship all your dependent libraries with your executables and wish to ensure that things JustWork(tm) after installation, in this case use DT_RPATH.

But why then RPATH got deprecated in favor of RUNPATH?

When DT_RPATH was introduced, it had precedence over all other parameters. This made impossible to override the libraries search path even for development purposes. Therefore another parameter, LD_RUNPATH, was introduced that has lower precedence than LD_LIBRARY_PATH.

More details can be found in the "How to write shared libraries" work written by Ulrich Drepper.

Chill's answer is exactly right; I wanted to simply add some color, from a recent reading of the glibc source ([master 8b0ccb2], in 2.17). To be clear, if a library is not found in the location specified by a given level, the next level is tried. If a library is found at a given level, the search stops.

Dynamic Library Search Order:

1. DT_RPATH in the ELF binary, unless DT_RUNPATH set.
2. LD_LIBRARY_PATH entries, unless setuid/setgid
3. DT_RUNPATH in ELF binary
4. /etc/ld.so.cache entries, unless -z nodeflib given at link time
5. /lib, /usr/lib unless -z nodeflib
6. Done, "not found".