I'm not sure about making the entirety of spells exclusive based on choice. While it could add a deeper of element of strategy to the game, I.E., maximizing your effectiveness and efficiency given only two or three types of mana, it could also take away one of those elements. For instance, in the 'Choose Your Element' type of play, all you really do is climb up your specific tree. Time isn't a factor, since it's basically just going from A to B to C, etc etc. While you might do this anyway in a 'Generalized' system, let us say in the event you only have one Shard or one type of Shard, then at this point, it's a strategic decision, not a rail-roaded game element.
But let's look at the event where you have multiple shard types. For arguments sake, we'll say my idea of Double-Cost for Off-Type, and no use of Diametrically opposite mana types, is being used.
Now, let's also say you have both a Fire and a Water Shard. In a 'Choose Your Element' situation, one of those two shards will be totally worthless. However, using a 'Generalized' casting system, you have the option to use both of those shards to their full potential, turning both into extremely valuable resources, and adding to the strategic and tactical elements of the game. In the first example, it doesn't matter if you let the 'Off' Shard get taken, since it's doing you no good, but in the second example, it's still a valuable and important resource, even if you can't use it right at that particular moment.
To add even more complexity to this, let's add a third example with the following parameters:
You've started the game specializing in Fire Spells. You have one Shard of each, Fire, Water, and a third element, take your pick.
Now, in a 'Choose Your Element' style, at most, only 2 of those 3 Shards would do you any good. Again, it makes a shard worthless to you on both strategic and tactical levels. There's also the potential that the third Shard may still end up being worthless, still leaving you with just one Shard.
Using a 'Generalized' system however, even if you can't use the Water Shard right away, it may come in handy later, say, if you need to counter an army of summoned monsters that were brought into being via Fire Magic. I.E., Ifrits, Fire Imps, what-have-you. Even though you may need to research a spell or three first, it's still being useful to you, just by being there and accumulating mana. This doesn't hold true in a 'Choose Your Element' style of play. The third Shard is immediately useful, operating as an off-type Mana source, allowing you to cast the spells you already have access to, and that aren't Diametrically opposed, with more ease.
Now, personally, I think the exclusivity behind Life and Death, in just about any game, is silly. Life and Death are intertwined, more so than Fire and Water, Air and Earth, or any other Element/Opposite Element combo that can be thought of. The only reason I tolerate it is because it's not simply about Living and Dying, it's about either everything living, or everything dying. It's about the battle between the Living and the Dead, or those that want to bring Death to everything, as opposed to both just being another part of the same cycle. This is a common theme in many games, and while I feel it is over-done and absolutely cliche, the fact is, given the ability to choose between either Life or Death adds a certain flavor to the game, and, in most cases, clearly defines the Protagonists and Antagonists, at least as far as any single player content goes. At the same time, it also helps to define play-styles. While neither Life nor Death is inherently Passive or Aggressive, each type offers a myriad of different ways to reach the same goal.
I.E., Life Magic Man needs more army, so he uses his "Inspiring Rally" spell, immediately recruiting more low-level soldiers to his cause.
The flip-side: Death Magic Man needs more army, so he uses his "Raise Dead' spell, immediately summoning some low-level Skeletons to fight for him.
Same out-come, different means. See what I mean about flavor and definition?
Let's complicate things further with a more in-depth example.
Life Magic Man is underpowered going into a battle. Not by much, but enough. So, he uses "Bless" to give his troops an edge, helping him to the obvious goal of winning the battle. It's still not quite enough though, so he uses "Angel's Presence" to give his Blessed troops a portion of the Angels power, and goes on to win the battle.
Death Magic Man is underpowered going into a battle. Not by much, but enough. So, he uses "Fear" to demoralize the enemy ranks, helping him to the obvious goal of winning the battle. It's still not quite enough though, so he uses "Smell Fear" to bolster the zeal of his troops against the frightened opponents, and goes on to win the battle.
Again, same out-come, different means.
And as an end note... I gotta stop using walls of text when I write... >.>