Ignore Dravic, he's quite simply wrong. Throwing from horseback is not only viable, it can be quite effective. However, it will have variable success against infantry, it is more effective in killing horses, and I will not pretend that it is easy or enjoyable all the time. Like all throwing, it is a joy when it works and a pain when it doesn't, though it is far more survivable to fail from horseback. I ran a very similar build for a long time, though I went with 2h instead of polearms.
3 HA is enough for horse throwing, it'll keep the reticule within throwing norms and balance out damage when you factor in speed bonus. If a pikeman is holding still and you charge straight, you will be able to hit him fairly often, circling outside the range of the pike you should be able to get him eventually as well.
While horseback throwing isn't the most popular build, it does have a good number of skilled adherents. I'd try and find one to learn from, horse throwing has a few tricks that can be taught and make you much more effective.
Oh, and this build will allow you to kill horse archers who aren't plated with relative ease, if they aren't very good. You'll have a fighting chance against the rest. That alone should be worth it.
