I love it when people with vested interests down vote on HN.
If Thunderbird or Mail.app advertised itself on emails you send out by default, people would be up in arms.
Do you see Gmail doing that? No. And even then, these three programs are free.
If I pay for Sparrow (and I did, and it was disappointingly slow and had poor UX -- but it sure did look pretty in the screenshots!), I don't want it to advertise itself to everyone I email -- because I'm not here to do your marketing for you. Let me opt-in to your poorly thought out self-promotion instead of pissing me off.
It's a new pricing model. I think they're calling it "shareware." The idea is that they release "free" versions of software that are almost fully functional but have small nags or annoyances to encourage you to purchase if you enjoy using them…
I apologize for missing the point. You're right; that is annoying and no, they should not do that by default.
Side-note: I just re-read my comment and I sounded like a jerk. I would also like to apologize for that. The sarcasm didn't age well (to my ear anyway).
Edit-- I just confirmed for myself that it does at least in Sparrow Lite http://dl.dropbox.com/u/7400069/sparrow_lite_signature.png