Wh atis the point of "as one"?

[Alexandros Satorum]

Would it not be just easier to gie the horseleord ride by attack as a bonus feat?

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As One (Ex)

At 9th level, a horselord gains the benefits of the Spring Attack feat so long as she remains mounted. The horselord uses her mount's movement for this action and neither the horselord nor her mount provoke attacks of opportunity from the target. When making a single attack with a one-handed slashing weapon while using the Spring Attack feat, the horselord treats her mount as charging. This ability can be used to qualify for other feats that treat Spring Attack as a prerequisite; however, the cavalier can benefit from those feats only while mounted.

[Akerlof]

Ride-By-Attack requires you to be charging, which requires you to move in a straight line and cannot go through any obstacles. As One lets you go through friends, difficult terrain, around corners, or literally ride circles around your enemy if you feel like it and still gain the benefit of charging.

It's kind of really good, probably not as good as Wheeling Charge (and/or Dragon Style for your mount) with a lance, but it makes non-lance builds pretty darn effective and would be amazingly OP if you could figure out a way to skirmish in this game.

[Mikko Kallio]

Ride-By Attack means you have to move in a straight line, and max movement is mount's speed x2. Spring Attack has more options, though max movement is only your mount's speed x1.

So, in short, this ability gives you a more interesting option than cavaliers normally get. Whether it's useful really depends on the build, of course...

[Are]

It's technically not the same; with RBA, you are charging, while with "As One", only the mount is charging (although with the recent discussions regarding mounted combat, I'm not sure these are different any more).

I agree it might be simpler to use RBA for the ability, but considering the previous ability grants Mobility while mounted, it's thematically appropriate to grant Spring Attack.

Edit: And considering the point the above posters made regarding movement (straight line vs not straight line), it's different, too.

(Interestingly, while "As One" says you can use the ability to qualify for feats with Spring Attack as a prerequisite, the "Sand Storm" ability doesn't say the same for Mobility; since all the feats that require SA also require Mobility, this seems like an oversight).

Also, as an aside, it's a bit annoying/confusing that there's both a "horselord" archetype (for cavaliers) and a "horse lord" archetype (for rangers), as well as a "horse lord" trait. I really wish Paizo would stop using the same (or virtually the same) name for different things.

[Alexandros Satorum]

Ok.

But then, for soem reason if you use an one handed slashing weapon (something that the archetype encourage) you have to move in a straight line.