[Sin]

For an online poker thing it would seem to be a pretty accurate marketing tactic.

They're into gambling after all.

They're gambling that the people they send these to, will be curious enough (or annoyed enough) to visit the site to find out what the bonus offer is.

Lets face it, how much junk mail do we get where we see the offer prominently displayed right there, think "I don't need that" and chuck it in the garbage? You've obviously held onto these items for a start, and I suspect that the number of people who are curious about the offer will far outweigh the number of ppl who aren't.