Crit/MR Pro's and Con's for Open World PvP/Closed Duels.
In the beginning of the patch (4.0), there was a large push by the Templar community to use Crit/MR sets to deal with the influx of gunners and bards early on in the patch. As the patch wore on it's effectiveness is still fairly strong, however it's met with flaws.
Crit/MR assumes you have the upper hand on someone, ie. You have AA and basically all CD's ready. It's a fairly long cool down, you're probably going to have it once for every fight you get in open world PvP, assuming there's a large enough delay between each fight that is.
Crit/MR also assumes you're substantially more geared than your opponent(s), or in the event that they're retarded, you're going to have substantially easier time with them. For example, a Sorc with Supp of Focus, to deal with this you need to have a fairly extremely geared MR Set ready for them or try and live through their CC phase which is made substantially easier with Stubborn Spirit. However if the sorc was without it, you can fairly reliably manage to simply burn them down without too much of a hassle with or without AA in some cases.
Magic resist is a ubiquitous Stat, it's potentially viable against almost every single class in the game in some way shape or form. Every class in the game has one or many skills which rely on having some form of potential MA component, which unless they're actively slotting it can be resisted. Magic Resist is more effective against some classes than others, however in terms of Open World PvP as you will more often than not be faced with multiple different classes, sometimes even all at once, it's nice to have the ability to still be able to prioritize more important targets that you need to be focusing down when shit hits the fan, since it's hurting you a lot less than others.
Crit/MR makes you a bigger threat, an annoyance if you will, typically if you see a Templar running at your group the best way to deal with them is to simply bury a CC on them and then pressure/CC their Cleric, but if you can't CC the Templar very easily and it's still running at your group and it jumps all over your Cleric while their group starts steamrolling yours.
Templars in Group PvP are a force multiplier, in that BG suddenly makes even the smallest group a substantially bigger threat than one without a Templar. Given the ability to prevent yourself from being stopped or prevented from playing a crucial role to your group should be a large priority you have at hand. Crit/MR allows you in addition to AA potentially resist an otherwise group-lethal Sleep Storm or Fear Shriek.
Multitudes of additional points can be made on a class to class basis, all of which are beneficial when factored in that most classes now have moved away from Slotting MA due to the fact that most Clerics have swapped to Block Sets.