It is designed for use outside of cities and arenas. And dual-specs will only be available at level 80. So the main use is raiders.

The most likely use of dual-specs will be raid vs grinding, raid vs PvP, and so on. Most of those can easily be changed in a city, before a raid.

There are exceptions. For example, switching from DPS to tanking for a specific encounter that requires extra off-tanks.

But far more interesting is that summoned Lexicon of Power will make it far easier to apply new glyphs mid-raid.

So perhaps we will see greater use of “situational glyphs” – glyphs that might be very useful on a specific boss, but are not currently used at all because they are so inconvenient to swap?