First time I've heard about this, but it doesn't mean it's not well-known.
Sounds a little similar to OATH HOTP / TOTP. OATH is based on RFC 4226 and seems at least at a first glance more robust. For example it uses HMAC instead of simple hash (motp uses MD5). OATH was also adopted by products like Yubikey and recently Google Authenticator.
Based on these very shallow facts, I'd say I would personally prefer OATH over this seemingly-bespoke solution. It would require a deeper analysis to really flash out the advantages/disadvantages of those schemes.
I hope this answers your first two questions.
As for the reasons for choosing it - One Time Password (OTP) not only protects against eavesdropping over the communication line. So whilst SSL will ensure the password (or one-time-password) is transported securely, OTP ensures that even if the password is lost/stolen, it cannot be re-used. It's unique for every session. So even if someone looked over your shoulder when you typed your password, since it's a one-time password, they can't use it again.
There are some other advantages to OTP schemes, in this case it acts as a 2nd factor authentication. Not only what you know, but also what you have. In those mobile-based OTP schemes, it's usually some secret key that is stored on your phone and is used to generate the password. So an attacker will not only need your password, but also your phone (or the key on your phone), in order to authenticate as you.
Most authentication schemes are complementary to SSL and wouldn't replace it. Even if the authentication process is secure, there's usually other data being exchanged, or some kind of session established. For example, consider logging into your bank account, and then viewing transactions or transferring funds. Protecting the authentication alone (e.g. via OTP) is not enough. Same goes to just protecting the connection (using SSL without authentication). To protect it better you'd usually combine SSL with some form of user authentication.
Regarding your concern about storing PIN/key in reversible form rather than using a secure-hash. I would say this is probably an acceptable trade-off considering what you gain. OTP gives you great protection against cases where the static password might be leaked (including phishing, over-the-shoulder, people writing down their passwords, social engineering etc). Even if one OTP password is leaked, it limits its usefulness -- it can only be used once and within a limited timeframe typically. The downside is that you have to make sure your authentication server (and to some extent user devices) are well-protected. If your server is your weak-spot, then, I'm not sure secure hashes alone are going to be enough to protect you anyway. Plus, combining it with SSL and a standard (hashed) password could really boost your protection even further.