Shared library .so-version bumps

They cause a lot of work, many programs need to be re-compiled.

Are they always necessary, or whims of the programmers ?

Here's one explainer that I saw recommended on StackOverflow:

Dissecting shared libraries

Short answer is that they're necessary if the newer version of the library is incompatible with the old one.

Note that the versioning also allows you to keep multiple versions of the same library, as is pretty common on Debian-based distros.

Originally Posted by Toutatis They cause a lot of work, many programs need to be re-compiled.

If it's too much work, you should switch to the stable version of Slackware (14.2 at the moment). That shouldn't run into library updates like -current does (since -current is the development version of Slackware and updates like this are to be expected during development).

you need only to recompile usage of shared libraries if the shared library has a new ABI that makes it incompatible.
if this is for security/stability reasons, you want to recompile apps using a lib anyway.

if its 'just for features' you could install them in parallel, like Qt4 and 5
works with Qt somehow, many libs make it harder to do this

As you can see the general answer is: sometimes yes, sometimes no. It depends on the situation. Do you have a specific case to discuss?

I just remarked that some libraries are updated rather often without .so-version bumps.
For some others, like poppler, there is always a .so-version bump. I wondered if that is always necessary (did ABI changed so many times ?)

Originally Posted by Toutatis I just remarked that some libraries are updated rather often without .so-version bumps. For some others, like poppler, there is always a .so-version bump. I wondered if that is always necessary (did ABI changed so many times ?)

This is solely based on what the developers of those libraries decide to do. Many times, many developers try to keep things the same so they don't break programs, other developers will make whatever changes they feel they need, even if it requires an ABI bump.

Originally Posted by pan64 As you can see the general answer is: sometimes yes, sometimes no. It depends on the situation. Do you have a specific case to discuss?

Gee, I wonder what that might be...