I'm not sure
exactly what you're referring to; the -soname ld option is used to set the value internally to a library that that library is known as
when building a library, and is used as part of ld.so at runtime to find the appropriate .so file to run-time dynamically link against.
The issue I described is during build time of a program that uses a shared library. Given this list of available libraries:
lrwxrwxrwx 21 Apr 20 2011 libevent.so.2 -> libevent.so.2.1.3
-rwxr-xr-x 106656 Aug 20 2010 libevent.so.2.1.3
lrwxrwxrwx 21 Jun 3 2012 libevent.so.5 -> libevent.so.5.1.3
-rwxr-xr-x 277760 Jun 3 2012 libevent.so.5.1.3
lrwxrwxrwx 21 Jun 3 2012 libevent.so -> libevent.so.5.1.3
there is no way to use the -levent option to gcc or ld to specify you want to link against libevent.so.2. If you just give it -levent, it will
always link against libevent.so, which as these symlinks show, is pointing to libevent.so.5.1.3 (which is actually libevent-2.0) — this is documented in the ld man page under the -l option as for how "namespec" is resolved. Unfortunately, the APIs are different between 1.4 and 2.0, which is why this is necessary: not everyone who uses libevent has updated their code to use the new libevent API, yet we still want to use that software.
The "fix" is to move some kind of "logical" version number into the filename (although, which is "logical" and which is "physical" becomes ambiguous), like so:
$ ll libevent[-.]*
lrwxrwxrwx 21 Apr 20 2011 libevent-1.4.so.2 -> libevent-1.4.so.2.1.3
-rwxr-xr-x 106656 Aug 20 2010 libevent-1.4.so.2.1.3
lrwxrwxrwx 21 Jun 3 2012 libevent-2.0.so.5 -> libevent-2.0.so.5.1.3
-rwxr-xr-x 277760 Jun 3 2012 libevent-2.0.so.5.1.3
lrwxrwxrwx 21 Jun 3 2012 libevent.so -> libevent-2.0.so.5.1.3
so you can obtain a linkage against 2.1.3 by specifying -levent-1.4. This "encourages" one to use the 2.0 API by using libevent-2.0 if you specify a bare -levent. The -devel package with the header files will usually be for the latest version anyway, complicating the build process on a single machine with both libraries available (you'd have to build against a local/non-system-include-tree copy of 1.4's headers). I know some redhat/centos packages do do this (which is why I have both libevent-1.4 and libevent-2.0 on my system, but I don't know how those packages' specifically do this.
The other way around this is to explicitly link against the full path /usr/lib64/libevent.so.2.1.3 and not use the -l option at all for this library. This works in part because of the -soname option you pointed out. While this works, and produces the same output if one could use -l to select the correct library file, most of the build tools we have at our disposal prefer to use -l to select libraries at build time rather than full paths (because if you use -l to select a system installed library, ld does the work of searching its cache and standard paths for you; however, you can get around this with the :filename version of namespec to the -l option). This means that some programs' build processes would be different on systems that have only the old, only the new, or both libraries on the system, and a bunch of programs build processes might need to change once the new version comes out.
The problem here is that -l only lets you specify the name, not the version to link against, under the (arguably well-founded) assumption that old versions should not be used for new builds but you need to keep them around for already-built programs to link against (dynamically, at run time).