Defense Mastery Tree is really outclassed by the Offense tree on most champions
9-21-0 is a useless mastery combination and has been since the beginning of the season. The only champions that you should be going 9-21 on are full tanks, and even then in some cases 21-9-0 is better.
21-9-0 is better on Marksmen, mages, assassins, most fighters, while defense tree is pretty much only good on tanks and a very few select fighters.
The main issue is that the offense tree has so much multiplicative scaling while the defense tree really doesn't. 5% bonus damage to low targets times 3% bonus for 21 points times the multiplicative 1% for 3 stacks on spellweaving/bladeweaving, along with the best mastery in the game (dangerous game). Defense tree has mainly flat masteries and the ones that are multiplicative are extremely niche (-10% on crits only), the worst mastery in the entire game (runic blessing), the overnerfed perserverence mastery, as well as the fact that defense in general is really outclassed by offense right now makes the defense tree really underwhelming.
Champions that used to go 9-21-0 in Season 3 and even more champs that used to go 9-21 in Season 2 like Jax, Irelia, Darius, Lee Sin, Jarvan, Olaf, Lee Sin, Warwick, etc, go 21-9-0 because the defense tree does almost nothing for late game since defense is pretty useless late game regardless, and not going 21 in offense makes their offense fall off VERY hard. The main issue here is that BEFORE, the %pen mastery (10% and 8% in Seasons 2 and 3, respectively) was accessible with only 9 points in offense, meaning that tanks/bruisers actually had a late game if they went 21 in defense. Now that's not the case.
In Season 2, the defense mastery was standard for bruisers and tanks, while the offense tree was standard on mages and marksmen (and the very few assassins that were used), but now the balance between the two is very skewed. The fact that Riot has no immediate new masteries for Season 5 shows that the entire tree won't be reworked for a while, but will some small changes be done to affect this?