As I see it, the only effect of having three sites instead of one is that questions in Server Fault and Super User are seen less and answered less.
This just causes audience fragmentation and dissipation for no reason.
In addition it causes useless friction and waste regarding the endless issue, where does this question belong? Belongs-there, belongs-here...
The site comes with a powerful tag filtering feature anyway.
Those three sites have very similar audiences, questions, and expertise fields.
Take for example:
- Where is the limit between writing macros and programming (Super User or Stack Overflow)?
- Is configuring rewrite rules for Apache programming or administration (Super User or Server Fault)?
- Is advanced registry configuration administration or power-user tasks (Super User or Server Fault)?
- Is writing Bash scripts programming or server administration (Stack Overflow or Server Fault)?
Feel free to add your examples in the comments...
I think the answer is that most questions fit in at least two sites and what matters is not the issue itself but what was the bigger issue the asker was trying to solve.