hard to say which tools users have running which are
built against MySQL 5.5 and not part of fedora and
if they break, and yes for users this is a topic beause
the distribution / OS for the user is a platform for
his work and tools and not the one and only

Ah but the context of the feature proposal and of this change is current MySql 5.5 in F18 to MariaDB in F19 ...

You keep bringing up the bump needed in 5.5.10 but that's out of context for this - anyone running against less than 5.5.10 would not be relevant to this discussion (as they'd have a similar issue updating to any MySql release that is 5.5.10 or higher) and that would not be a F18 to F19 upgrade...

For users who have tools that work with current the current MySql 5.5 release there is full compatibility whether you mean by API, ABI, server or client libraries or files... it is only those users that are in context for this discussion.

If you have a specific example of this not being true then I'm sure the MariaDB guys would love to see the bug report to fix that - otherwise this is just a matter of discussion outside of the scope of the change and based solely in the hypothetical.