According to Mark Suster's earlier article -
http://www.bothsidesofthetable.com/2012/09/05/the-truth-abou..., there are 2 main downsides to convertible notes from a founder's point of view:
1) If an investor invests $500k at a $4.5MM cap, he signs up to get 10% of the company assuming the Series A priced round will be at a higher valuation. But, if Series A is at a lower valuation, say $2.5MM, then the investor gets 20% of the company. However, if there was a discount associated with the convertible note then the note converts at 80% of $2.5MM. So a down round is really bad for the founder. But I can't imagine a down round being much better for a priced round.
2) If the Series A is $3MM at $12MM pre with 1x liquidation preference, the Series A investor gets 20% of the company. The seed investor who invested $500k will have his shares converted at the $5MM valuation. However, he will end up with a 3x liquidation preference or $1.5MM in liquidation preferences. I think this is okay as long as you raise a few hundred thousand dollar convertible note seed round. If the convertible note seed round ends up to be in the $1MM range I think this can become an issue for the Series A investor which is Suster's main point.
Convertible notes have benefits like high resolution financing, less control & no board seats. I still think convertible notes are the way to go if you are raising a few hundred thousand dollars in seed. But if you raising a seed in the $1MM range, it seems like priced rounds are preferred.