Mastery vs Variety

An interesting concept that often comes up in the context of elemental magic/skill systems is whether the player characters should have mastery of a single element of their choice or whether they should use variety.  In many games this choice is truly non-existent as either the element chosen is just based on what the enemy is weak to, if any, or which effect looks prettiest.  Some games allow the player to enhance, via skill points or equipment or levels, into a specific element.  However, under these circumstances, the player only has one correct choice, which is to master one element that seems useful.  It is rarely worth it to try to put skill points or whatever into multiple categories because then the player's power is just simply lower than it would have been otherwise.  The elements often have limited meaning other than, in this specific fight you'll wish you had one over the other.

To this end I am thinking of going with the variety approach, but with the simple caveat of the elements being asymmetric.  Hearth and Tundra will not be opposites, they will cause different effects that will give the player a desire to use one element or the other under various circumstances.  For example, a multi-turn tundra spell will enhance the party's healing spells while it's being cast and then poison the foe when it goes off.  Fire spells by contrast will enhance the party's water resistance while being cast and reduce the enemy's attack when it goes off.  These two effects are very different, and will hopefully lead to players debating how much "mastery" via equipment they want to place in any given element.