A lot of buyers dont leave feedback nowdays. Some of them because they were slightly unhappy with the purchase or some aspect of it. Sending any kind of note that insists the buyer leave positive feedback and/or good dsrs is pretty much going to encourage the slightly unhappy to be even more unhappy and probably impact your feedback the wrong way.

Buyers want their item, they want it in stock, they want it delivered on time and they want it in good condition....thats it. The last thing they want to read is a poor me message about hard hard it is for small sellers on ebay and how the big guys get all the breaks.