It seems to me that:
- The sp1 tag is useless on its own - it does not atomically represent a concrete, independent thing. Tags should not really change in meaning depending on what other tags they are used alongside. There's no value in, for example, searching for recent sp1 questions.
- The fact that an issue pertains to SP1 of a particular Tridion product is very relevant and can have considerable implications for the validity of answers (e.g. answers making reference to the Import Export Service in 2013-related questions). As such, 2013 and 2013-sp1 are meaningfully different and should not be synonyms.
In essence, the fact that the sp1 exists suggests that people feel that it is relevant information to include - which it often is - but on its own it is not useful (what would the tag description say; "This tag refers to service pack one of some unspecified version of Tridion?")
It would be more useful to allow 2013-sp1 to be used where the question refers to issues and features specific to SP1 and not allowing it at all seems an unnecessary restriction since the fact that a question refers to SP1 is potentially useful when searching and filtering questions.
On other StackExchange sites, similar scenarios are handled well by allowing both tags - in this case 2013 and 2013-sp1 - to be used in conjunction with each other, which provides the ability to search/filter both by the "broader" topic of Tridion 2013 and more specifically-targeted searches for 2013 SP1, while still having each tag be independently meaningful. It is certainly not detrimental and there are plenty of scenarios where it's useful.
So, I propose the following:
- Remove the link between 2013 and 2013-sp1.
- Replace combinations of 2013 and sp1 with 2013-sp1 (and likewise their 2011 counterparts).
- Remove sp1 from any remaining questions (as a community editor task).