Well, at my table I'd rule that you can't be kinda pregnant, dead or concentrating. . .if you are not concentrating within the round, you're not holding the spell, and it doesn't work, if you are concentrating, then you pay your 50%.
To offer a strictly RAW example,
Round 1
the party is on a battlement, Stealthy Pete is picking the lock on the tower door while everyone else is behind cover, due to a crossbowman on the opposite tower. . .Blue Mage casts Deflections II on Sneaky Pete, the archer fires, and the shot is deflected. . .In deliberate Sneaky Pete pops the lock.
Round 2
Now, if the Blue Mage wants to keep up the spell, he needs to declare he's concentrating, which right off the bat should take up 50% action, even if on snap phase Pete pushes the door open, and the next crossbowman in the tower shoots Pete right then in snap, triggering a 2nd deflect and ending the spell. . .it's not like concentrating for a phase costs less than concentrating for 2, or the whole round, it's just 50%.
That said, since the spell is done in snap phase, Blue mage could then cast in the latter two phases of the round since he has not cast a spell this round. . .as long as it was another instant spell since he has only 50% left.
Still even assuming you house ruled to allow it, using your example above. . .if Blue Mage cast Bladeturn on round 1 Deliberate, rather than Round 2 Snap, by the same logic as the RAW Deflection II above, pre-casting he'd be spending 50% action concentration carrying it into round 2, rather than 10% action just casting it in round 2. . .the major difference being that the pre-cast-and-carry would technically allow the casting of another instant spell in round 2, while just snap casting would leave the blue mage 90% action to do something other than casting (which for a mage is usually some variation of moving and looking, perhaps a full parry.). It also guarantees the spell is on before the attack, no worrying about initiative.
So I guess there would be some upsides, though it's not pure upside, since you are spending 40% activity for the privilege of going first and being able to cast another spell in that round. . .
I honestly can't say that it's ever actually been attempted at any table I've played or run though, and the people I play with can be lawyerly cutthroat. . .reading over that errata list is often an amusing trip down memory lane of "Oh yeah, remember when we allowed that!?!"
In my experience, if presented with a round up front the mage will just let someone else stand in front who has the OB to parry with, and choose to go with the best defense being a strong offense. . .like tossing a fireball in the room, or having someone pull the door open so they can snap blast the guy inside over their shoulder outside melee range, or cast a sleep in there or some such. . .
Then again, I'm sure there are people who'd jump on that if a GM allowed it. . .and it's certainly not the killer ap the original poster was talking about. . .the pre casting, lasts until triggered, don't concentrate is definitely pure upside and would rapidly get out of hand if allowed.