Disagree. Jax deals 50% magic and 50% physical damage with his skills and every 3rd hit from his ultimate dealing 300 magic damage, not to mention leap+empower harass. Buying either form of penetration is not particularly efficient on him. Front-end attack speed gets your stacks on quicker, exponential like kwj said above me, so you might get 10 stacks of your ult and 8 stacks of rageblade in 7 seconds instead of 12 seconds if you got some front end attack speed. Also allows you to jungle much smoother at level 1 if you choose to jungle jax.