I personally love having shield skill on my lancer.
With your build it sounds pretty pointless to be using a regular lance/light lance/heavy lance since you won't have much power strike (I'd recommend at least 12 str for 4 powerstrike).
I honestly think your build is terrible though, I always recommend only going the riding you need for the horse you want to ride. Sure having 6 riding when I jump on a rouncey is nice, but it's not like I'm going to fly up on someone with 3 riding on the same rouncey, unless they put themselves in a terrible position (i.e. one where they can't get back to their friendly infantry), having that extra riding is pointless.
You're going to NEED a shield if you use a couch only weapon, otherwise you won't be able to cancel the couched lance until the 8 second timer runs out. Also I don't think this couch only lances can block (unless that's from me playing too much single player mods).
The shield is very nice for blocking projectiles, also it's nice for blocking someone's couched lance if you catch yourself in a bad position.
I'd recommend this build (so you can ride an arabian), but I wouldn't recommend going 8 or 9 riding. If you do get a shield with your 9 riding, you're probably only going to want 2 points in it (there's some decent 2 skill shields) since you're going to need to be putting points into athletics and power strike.
7 riding (at level 30) build, always plan for a level 30 build because I'm assuming you're going to retire at level 31 (if not, then plan for a level 33 build since it's not too difficult to achieve). Using the character calculator (the WPF is not accurate on this site):
http://infinitum.dyndns.org/crpg/calc.htmLevel 30 (4 420 577 xp)
Strength: 15
Agility: 21
Hit points: 50
Skills to attributes: 2
Power Strike: 5
Shield: 4
Athletics: 7
Riding: 7
Weapon Master: 6
You can either do 6 points in weapon master, or put 5 into IF and the other 1 into shield (that's up to you and depends on what you want, if you're using a couch only lance, then I'd suggest going the IF route, but I love my WPF, it helps with speed and makes the weapon hit harder, not to mention you have less chance of getting hit with upkeep on weapons the higher your WPF in any weapon skill).
With 6 WM and 21 agility at level 30, you can have 154 WPF in polearms which will help with thrusting lances, and when you get dehorsed and fight with a polearm (WPF helps with couch only lances by giving more damage, but they do a lot of damage already, and it's not going to be noticeable enough to use versus putting those points into IF).
I always put points into WM before IF because I'd rather have an offensive skill (weapon master and putting points into WPF) than a defensive skill (Iron Flesh giving more HP).
If I haven't convinced you yet, and you're hell bent on going 9 riding, then I'd suggest this build:
Level 30 (4 420 577 xp)
Strength: 12
Agility: 27
Hit points: 47
Skills to attributes: 8
Power Strike: 4
Shield: 2
Athletics: 8
Riding: 9
And I'd HIGHLY recommend that you use a great lance.
Since you already have 23 agility, my 7 riding suggestion probably won't work (I would not recommend respeccing at this point), so my suggestion would be to go 8 riding/24 agility with this build:
Level 30 (4 420 577 xp)
Strength: 15
Agility: 24
Hit points: 50
Skills to attributes: 8
Power Strike: 5
Shield: 2
Athletics: 8
Riding: 8