If fact, I am asking the question posed in the title:
WHY do I need Multiplicity... over Synergy?
To be honest, if synergy is working for you, you probably don't.
I've used both, and I like Multiplicity more than Synergy, and it has several good features (and some that don't really matter to me, and the one really good one that you have to update to Pro for). It has better polish than Synergy, and given that Synergy hasn't been updated in about 4 years, better support, especially for newer systems, as well. Synergy also has the odd bit of buggy behavior for me, and I've poked around in the sources for it, but haven't pinned it down yet.
That said, I don't use Multiplicity anymore. I'm still disappointed that Stardock has apparently decided not to try and support Mac / Linux functionality, though they originally indicated that they planned to do so.
If the only systems that you have are Windows systems, I would choose Multiplicity over synergy for the things that it has that Synergy doesn't. If, like me, you have multiple different systems (like my Win / Win / Linux / Linux / Solaris montage), Multiplicity really isn't a good fit (Synergy doesn't do all that well with Solaris for me, also, even though it's 'supported'). As a result, I don't use either (just a ton of xsessions for now).
If Multiplicity would get Linux support, I'd buy it again in a heartbeat, but in its current state, it's not the right tool for the job.
