I'll tell you why! For one, Rylai's just isn't enough cc when in team fights. Second with frozen mallet in between spells, spam your auto attacks on who your targeting. no matter whom it is (even if multiple targets) they will be slowed so miserably that they will shy from you in future battles.

CONS- I start with cons to show you that there aren't really any with this build. MR and Armor are your ONLY weak points, if you can use your spell vamp/lifesteal successfully in battle, and POP your shroud at the right time (usually after the fights been going on for awhile, try not to POP it right when you ulti in) and you will NOT die.

PROS- With Frozen Mallet, Rylai's and Trinity your HP is well above 3k, which is amazing for the BURST dmg you do. not to mention a whopping 75% SLOW chance when you focus spells and auto attacks on the same target. NOT even the fearless Rammus can run from you (and he will TRY)

I've seen in FAR too many occasions that Akali start with ONLY one buff and sometimes not even any.. this is just careless. with this build you start off over 20AP(gaining another 20 through champion lvl 18) Giving you your most important buff. BONUS magic damage. this is huge for akali! I've tried having my runes to approx. 20Ap with 12AD starting with both buffs and not purchasing an item. i've found however, that the 8.9 Spell pen is HUGE early game. Hence buying Long sword to give me BOTH buffs for ganks!