I always wonder why using swordsmen when I can use polearms, if both have pretty much comparable unit stats??

Swordsmen and polearms share same advantages. Same unit size, no need for formation like spears. Yes swords against spear have +1 attack, but polearms have armour piercing effect, which often gives more than 1 attack bonus in High and Late eras.

Also, swords are weak against cavalry, which is said to "always push back swordsmen"... As we know, pushing back gives enormous attack bonus to next attack. I read that from one of the guides from official medieval total war.

So why should I bother with swords when similar polearms are available??

ps: I read also from official mtw, that because polearms like spear are long weapons, soldiers at the 3th and 4th rank could still attack the enemy. If this is true, I wonder if it is still wise to have polearms in 2 ranks, as recommended by frogbeastegg in his Total Unit Guide. I said this is because, after 1.01 version, cavalry can now push back even spears, pikes and polearms.

I don't know if what I have read are true. So please someone correct me.