Take your pick:
- Secure
- Delivery/read confirmations regardless of carrier
- No worrying about 160 character limit
- Works when email is down (depending on the nature of the outage)
- Free if you have a BB unlimited data plan. Doesn't matter where your recipient is located -- you're not charged extra for international PIN. Though if you're traveling internationally you're subject to roaming charges unless you have an international BB data plan.
- Messages don't disappear into the ether or randomly get delayed like they do with SMS.
The real question is why aren't you PIN'ing and why aren't more of your friends using BB's so you can PIN with them?