I agree with both previous answers in a 50/50 ratio. There are some applications which will (or should) always been fringe-on-topic cases. Maya is one of these, and as such, should be synonymized with external-applications.
Some others, such as Make-Human, should have their own tags, because they are likely to be used along with Blender quite regularly (and in the case of Make-Human especially, it was once a Blender plugin).
This site is about Blender and as such, all questions and tags should be about Blender or anything related (general CG topics etc). Questions about other 3D software or applications are on-topic however, provided that they follow the guidelines outlined here - Should questions that compare Blender and <insert competing 3D modeling software> be allowed?.
If we encourage these unique tags, there is no limit to this and as such, anyone asking a question can easily create an accompanying tag such as zbrush, sketchup and maya etc. These questions are already borderline and one will also notice that they are usually very localized. Therefore, I don't think other packages need specific tags on this site.
I propose that we synonimize every existing tag (and future ones) that relates to another application under external-applications.
Blender is frequently used as part of a process e.g. to create models for game engines.
When the question is related on modelling, import, export or restrictions of available features on the target platform it would be easier to find (or filter) questions by a tag of the related software.
On stackoverflow there are currently aprroximatly 37000 tags. Why so many?
Because there are several programming languages for each is a huge number of frameworks/libaries in different versions available. As a long time SO user I often use the tags to drill down to find answers. Even a very basic control structure for-loop has already almost 10K questions.
Mostly the language java c++ is used as a primary tag, then the library spring stl serves as secondary classification followed a list of tertiary tags which give rough information on the issue conversion exeption errormessages etc.
How could this work on blender.SE?
Example of actual redundant use of tags:
Tags help to structure content, if someone tries to add interseting questions to the favorites he will probably not find them again if the number exceeds a few hundred. Organizing the question by bookmarking in the browser could help, but the structure information wouldn't be shared.
Therefore please let us have more (non-redundant) tags.
I think that it makes sense to keep non-redundant tags for external software if that software is being used to compliment Blender in questions that use it. As stacker wrote, tags can be used to organize things in different degrees of hierarchy. For example, the tag
external-applications could be used as proposed, but adding the tag
unity could further refine a person's search. If somebody is interested in all external applications, they could search by that tag. However, organizing by specifics, like
unity, can be very useful if the user is only looking for that.
However, when not used to compliment Blender, redundant/unnecessary tags for external applications should not be kept, for example an application which exported a model to a standard format where the question is not regarding the opening of the model. Also, questions should not tag external applications in a way in which they are competing, for example "I want to do _________________ like in Maya" and tagging