In theory it could shorten the life of the SD as they have a finite number read/writes before they fail. Now whether it will cause the card to fail before you're ready for a new phone, well, I suspect nobody really knows for sure.
However, the dalvik cache is used for quick access by the phone to speed up some processes, moving that cache to the SD card is going to slow that down some. Of course we're talking about milliseconds here, so you probably won't really notice a difference.