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.
I am no fan of the defect system, but TBH I have always thought it wasn't the greatest idea for sellers to take the "here's my achilles heel, don't shoot it" approach. 
I understand the idea of needing to do something to try and defend against the potential effects of the new system, and the concern over the actions of some who don't understand what kind of effect their actions can have, but I agree that if nothing else, buyers don't want or need that responsibility - and in a perfect world, they wouldn't have it - but I honestly think shifting focus from the buyer and transaction to what the buyer thought of the transaction and how they review it is a self-defeating action.
A few years back, I stumbled upon this little gem, which is a particularly unique, manipulative and patronising guilt trip, IMO:
I don't encourage buyers to leave me feed back, you can never tell what they'll leave.
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