If you are an arcanist you can get some pretty significate mileage out of weapon swap. I use it regularly (honestly I am using it on almost every arcanist I am playing)
Put aetherscorched cleaver and Mythical ixillor’s rageflame in your weaponswap slot. Duration and cooldown of temporary buffs (mirror of erectos is one of them) is determined at the moment of casting. So if you use mirror regularly as part of your rotation it is possible to swap, cast mirror, swap back and get 5 seconds of invulnerability instead of 3 for relatively low drawback (drawback is that you can’t use aura components on your weapons).
Best example would be aether spellbinders like [1.1.8.1] [HC] Scion of the screaming Veil Spellbinder ( SR 85, 6 min Crucible, 5:05 fastest, Ravager of Souls) or The Outworld Devourer (1.1.7.1-1.1.8.1) - 2H Spellbinder (EVERYTHING+). They don’t use seal of might and really enjoy cdr and mirror duration. For example, Retal Abuser’s spellbinder has 15.2s cooldown of time dilation and mark+mirror lasts a total of 7.6 seconds. If you play exactly the same build but add axe&offhand on the weaponswap you can swap any time you want to cast mirror or trigger time dilation (better use bone harvest as a TD proccer then), this means that you end up getting 12.3s TD cooldown and 9.7s duration of mark+mirror. With eternity relic the build already can chain them infinitely but even without it, raising uptime from 50% to 80% is amazing. Keep it in mind that it takes a while to get used to it.
Similarly, sorcerers can get near perfect uptime on blast shield/mirror and you can make a troll build that has permanent uptime on blade barrier/mirror using the same trick (it is very awful to play though, would not recommend for hardcore even though it can technically facetank any damage because of permanent 100% absorb. But if you ever decide that you need a spellbreaker that can facetank callagadra, here you have one 
).
Even better example would be mad_lee’s build [1.1.7.0] Green Lantern: 5:45m Crucible Gladiator 150-170, easy SR 75-76 Aether Lightning caster Spellbinder [vid][c+][sr+]. The build has +13 to devastation yet isn’t using it because it is a 2handed build. Well, you can probably see where I am going, having axe+offhand on this build will also allow you to use devastation (so you need to swap any time you want to trigger time dilation, mirror or devastation) on top of having almost perfect mirror/mark uptime.
Also if you decide to play it (they are fun and rather quick to get used to) I will warn you that I won’t advise you to overdo it. It may seem appealing to swap when you are casting mark of torment or any other ability with cooldown, but it’s not worth it, it’s better if the build uses it only rarely (all the described builds weaponswap only once every 10s and it’s very simple, swap->cast->swap back).