Riding piggyback in combat???

[Artoomis]

Ride by attack leaves your mount open to AoO's. It just means you don't get attacked. I'm not a huge fan of it myself....

No, Ride-By Attack protects both you and your mount.

The feat states "Prerequisites: The character does not provoke an attack of opportunity from the opponent that the character attacks," which refers to the character only, not the mount. You might think this means the mount provokes an AoO, but that would ignore the following:

The feat also states: "...the character may move and attack as with a standard charge and then move again (continuing the straight line of the charge)...," which also refers to the character's move, but of course it's really the mount that is moving.

Reading the feat as a whole, it's pretty clear that Ride-By Attack affects both the rider and the mount.

Of course, I wouldn't apply that in this case, as the "mount" is moving independently of the "rider."