I understand the gut reaction, but it's https! The real risk lies at each end of the connection. But then there would still be risk at each end even if they used something like gox's secret.
DO you not have faith in https?
There is a huge difference in using generated key/secret pairs from using the actual username/password pairs. For a starter, the former can be restricted to perform only certain operations, the latter can perform everything always. Even if some key/secret from MtGox or BTC-e were leaked, it could be the case that the attacker wouldn't be able to do anything interesting with them.
I can understand that the ability to disallow withdrawals for the key/secret type access can improve security.
But I guess what key is that in the case of the key/secret, at no time is data sent to the server, that if it fell into the wrong hands, that would allow someone else to create another api request. (They don't have the secret). However with the username/password if the server is compromised, the data sent by a user would contain enough data to make any api request they want.