Is there a conceptual limitation to have a clone stage function in Bamboo?
Stages group jobs inside plans. So technically they are cloned when the respective plans are cloned.

Which means that missing the ability to clone a specific stage should be just a UX/usability/functionality issue, not a conceptual one.

There might be a non-trivial implementation details when cloning stages - they may carry inside stage sequencing information, as in "pointers" to the stage objects preceeding and/or following them inside the plan "pipeline", which might be a bit more difficult to handle when cloning just one stage in the sequence. These would be easier to handle when cloning entire plans as the entire sequence of stages is cloned, there will always be a 1:1 relationship between the stages.