An evocation is an expression of the imagination, and an energy sword springing from your hand is probably one of the easiest things for any of us to imagine.
I got no problem with that, flavouring an evocation attack as a flaming sword is pretty cool and can easily be done. However, making it a lasting spell is not that easy. It is the difference between lifting a weight once or lifting it and holding it there for a while. Look at Harry, he is one of the strongest wizards out there, and even he has problems keeping up an evocation for long without getting exhausted.
But if you use it as flavour, you don't have to make the sword disappear only because he doesn't attack with it. If you do a fireball spell, the fireball appears, hits, disappears. If you say "hey, I'll do this as a "sword of fire" spell", you can have the sword of fire the entire fight, but if you attack with it, that would be a new evocation attack every time. That's the difference between narrative and mechanics. Yes, that limits the number of attacks you can do, but I think that's a good thing. It is an integral part of the wizard in my eyes.
Now don't get me wrong, I'm not trying to say "don't do this, it is terrible". I just think that trying to model something like this as a prolonged attack spell is neither practical nor balanced (and trust me, I tried). If you want to go the "generous GM" way, which is something I can totally agree on, don't use my claws suggestion, simply let him do a maneuver spell, tag it for effect and he has a flaming sword (or whatever else) to work with. If the weapon rating isn't too high, it is pretty much just a very easy to conceal weapon and it is pretty cool to boot.
He could do it as a rote spell and he wouldn't even have to roll, it would just *poof* be there as he commands. Or as an enchanted item. First use per session would be free, any subsequent summoning of the sword would cost him 1 mental stress, same as a rote spell.
But anyway, the thread should be about the principles involved, rather than my game in particular. As I said, I'm gonna playtest it and see how it goes.
That's what I'm trying to do. On principle, I'm saying no, there shouldn't be any prolonged attack spells. BUT, and here is where it gets complicated, there are a number of ways to emulate a prolonged attack spell by going outside the spellslingers toolbox.
The reason I keep coming back to your game is that, as I said above, there are a multitude of ways to solve most problems. Especially with the magic system, I think it is just more practical to leave it as is and model specialised magic as reworded powers. And I don't think it is bad to say "You know, you just can't do that with magic" now and again. After all, even if you can do a lot with the dresden world magic, it can't do everything. Might even be worth a compel every once in a while.