eclipse-plugin-dev has 63 questions.
eclipse-plugin has 1761 questions.
No, I don't think they should be synonyms. Eclipse plugin development is a different enough activity from just using plugins for development that I think a separate tag is justified. I strongly suspect that you'll find a lot of plugin development questions in eclipse-plugin though.
At the time of writing:
eclipse-plugin-dev has 162 questions, and has 34 followers.
eclipse-plugin has 4196 questions, and has 490 followers.
Selecting the 10 most recent eclipse-plugin questions:
Out of 10 question, only 1 was talking about plugin usage and didn't have another more relevant tag applied, 2 others were about plugin usage, but had related more specific tags applied. 7 were misusing the tag eclipse-plugin.
This raises the following questions:
I believe it would be a lot clearer to make eclipse-plugin-dev a synonym of eclipse-plugin, merging them, and updating the wiki to reflect this tag is about usage and development. Probably people who are knowledgeable about the development of plugins are also capable of answering generic problems with plugin usage anyway.