It seems the go-to for any AP champion these days is to get Morello and Sorc shoes as fast as possible _**after their core item.**_'Cause 33 pen right off the bat is just a great deal when every champion has lower MR than armor to start. It's enough to completely negate the MR of any Ranged champion, even.
Then there's the fact their first item grants all the resource and CDR they could ask for on top of a great passive.
It's a little strange that mages and AP assassins can just opt for a bunch of pen with essentially no trade-off, since the secondary stats they need are all in their first item. And with Morello's granting both HP and GW, it covers defense as well as enemy healing on top of the penetration. Then there's the fact that unlike armor penetration, there's a boot enchantment for magic penetration, making it even more accessible with no downside since it doesn't have some kind of balance trade-off like lower MS than other boots.
I don't think it's right for every mage and ap assassin to be able to just buy a bunch of penetration and have it be effective in pretty much every scenario thanks to base MR.
Should MR be increased, at least to the level of melee champs, across the board? Maybe. Though I think it'd be better to shift magic penetration _off_ of items with health like Morello and Liandry's, and off of boots, onto more offense-focused items. Probably an entirely new set of items.