For some Wikis there exist plugins for importing data from other Wikis.
I haven't found an overview or a matrix about every possible migration path.
My feeling is that - in the absence of a commong wiki markup - it's quite difficult for a user to switch to a different engine, unless there are specific scripts allowing one to. One tool you might want to take a look at is David Iberri's html2wiki converter. Using HTML as an intermediate format seems to me as the most sensible way to manually export/import content from one wiki to another. I asked David (nice guy btw) if he's planning to implement a wiki2wiki converter, but he looks quite busy at the moment.
I agree, that migrating is often a hassle, however converting the simpler syntax is generally pretty simple and can be done eg. in a short script.
Have a look at these scripts for an example:
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