You would have to go back to season 2 to understand the love of Wit's End some Irelia players still have. It was however so good that it was arguably the only ever other core damage item on Irelia. Build it to contend with a Vlad or teemo or to make the enemy unable to reduce most of your damage buy rushing a armor item.
Then in preseason 3 Riot removed an Item called malady and gave the MR shred passive to wit's end while increasing the cost and the AP ratio on auto attacks to Nashor's tooth.
In season 2 Wit's end had a cost of 2000 to 2200
40% attack speed
30 MR
42 on hit magic damage
Auto attacks increased your MR by 5 for 5 seconds stacking up to 4 times.
It now has 10 more MR and one more stack. It is not as good as season 2 but it is good enough to consider agaisnt hte likes of Teemo Vlad or even Sunfire Renekton.
It was cheap in season 2 but really strong for the price. You bought it for early to mid game power and lane dominance. It would fall off as the game went on since it was not the best buy for the item slot but it had reaso nto be bought.
Wit's end basically died for all but WW and a couple others when Malady was removed and the MR increase became MR stealing for too high of a price tag.
But we long time IRelia and Jax players are really excited at the prospect of it being a item to consider getting again.
And yes mixed damage is a big part of the reason why we wanted it back. while it is lacking the Universal answer of 42 it should be good enough to justify getting in some games now.