I can't find fault with much else in the Epic IG Codex, and look forward to eventually getting a Tank Army, but I have to agree about the Tank Riders.

Having tank riders safer than troops inside Armoured Troop Carriers just seems weird.

If they where cheaper, 75-100pts, and must take a 5+ save everytime the Tank is hit- they'd still be viable.

I'd attempt to move the Tanks close to cover every activation, disembarking the Tank Riders at every oppurtunity to take advantage of it (just like they would to root out hidden enemy anti-tank teams IRL)- they should ride the tanks during your activation but rarely be sat on them when it comes time for your opponent to shoot.
Their purpose is to act as a screen to enemy Engaging formations, by putting them around the periphery of the tank formation it forces the enemy to move into base contact with them rather than your tanks with better Firefight (again, just like they would to repel attacking enemy anti-tank teams IRL).

Their secondary purposes is to bulk out the formation.