The problem I have with adding a penalty for dying is what do you do for the other classes that don't die?
You know, the crafter and gather, they can go to higher rank recipes and nodes to skill up, and suffer no penalties for failure, so why should combat classes?

Honestly if you're dying repeatedly you're probably not high enough or geared well enough, and are likely killing too slow for the SP to be worthwhile, so it really makes no difference if people are fighting while weakened and dying repeatedly as they are not fighting effectively.