Losing the +5 for two free hands is not important. But if you're a semi, I'm assuming you have a weapon in one hand, so if the shield spell occupies one hand, you now have NO free hands. That means you actually have to MAKE an SCSM, which you don't with one hand free. That gives you a rather high chance of not getting a spell when you need it (by getting a near success or partial success), or even failing it. Also, it's likely that you're in melee, or at least in a melee environment. That's -30/-20 to static maneouvers (which an SCSM is), which REALLY makes you want to NOT have to make an SCSM in order to cast spells (note that being in melee or in a melee environment does not in itself force you to make an SCSM, but it modifies SCSMs if you have to make them). This makes it dangerous to cast spells.
So yes, for a semi (like the warrior mage), having one hand free is quite important.
We all play it how we like, of course. I'm very happy with Shield the way it is described. It gives you a +25 to ONE attack each round (if you're aware of the attack). Not necessarily better than, say, Blur, which gives you +10 against ALL attacks each round, whether or not you're aware of them, and that spell can also be used together with a real shield, and definitely does not occupy a hand.