What? No! Why? Nooooooo, no no no no...
Motivation? What? It's not that you are payed to contribute on the site... It's about helping others, why should you "reward" for being a good contributor decrease if you are having a time off?
This is a bad idea, an awful one, really not good.
Besides, if you stop contributing for the site, there is a chance that your posts might get outdated and down voted over time, why more measures that force you to be active? Contributions you do on the site are to improve the information available on the internet about Ubuntu, they are welcome in any form, any idea damaging those contributions would not be a good policy or act to improve that.
Make it better, be part of of it, in your own time. No one cares about karma in launchpad (at least not to be worrying about it), why should we?