Your spell defense is on until you end it, yes, and it isn't ever "used" up before then, so theoretically a 1000 mages could cast Detect GentleManLoser and were you a mage you'd be entitled to Counterspelling dice against each of them. And honestly, doesn't that really answer the question why some people feel that there should be a penalty attached? It's like the magical equivalent of always getting to be on Full Defense otherwise. It IS a balance oriented houserule, yes, but doesn't it seem a little odd that creating your own personal anti-magic zone is so much easier than sustaining a spell?

Anyway, my current GM plays under the standard counterspelling rules, and yes, my mage basically actively counterspells at all times. My old GM penalized much the way DireRadiant suggested and I actually had to think for a second whether I wanted to attempt to run a Jammer, two instances of Improved Invisibility AND actively counterspell at the same time. Whether that's good or bad is up to you.

If you do this, you actually aren't hurting mages as much as you are hurting non-mages. Since counterspelling unactively is always free, mages just wouldn't actively counterpell, and wouldn't declare counterspelling on their fellow shadowrunners(non-mages) because of the sustaining penalty. So in reality, the mage gets a little less, and the non-mages get a lot less. Active counterspelling does seem like it would require a mages focus and thus act as a -2 distraction penalty. This again, hurts non-mages more. There are really only certain spells that require active counterspelling, spells like mental manipulations and illusions. These spells are most likely to be cast on a non-mage rather than a mage, because non-mages tend to be more susceptible to these kinds of willpower resist spells. Mages almost always have a high willpower due to drain. A your fellow non-mage runner however, would do well to have those extra counterspelling dice from active counterspelling.

I know it seems to make sense to do this, and that at first you are somehow balancing mages. However, in the end, you just making mages more powerful, because now non-mages are more likely not to have any counterspelling. The only way to balance against this, is to allow counterspelling foci to remove the -2 penalty for all counterspelling.

You get a Magic+Intuition (3) test to see if you CS a spell you are unaware of.

Tarantula, ah yes! missed the LoS rule. I knew I was missing something there.

So as it stands, unless you lose a surprise test, you can always CS any spell effecting you, even when unaware.

Must get Shielding to add more free dice!

But I agree this is a litte too much, and a little too good!

So the quote from Street Magic;

Is just plain wrong? Unless they mean using CS to protect someone else...

Like most things relating to magic, it's whatever you (the player) want to describe it as. For one magician, it may be them projecting their aura around their comrades. For another it may be some crazy technomagical gizmo they cooked up that generates an antimagic field around those in range. For another it may be Dog watching over his friends. Whatever.

That's the best thing about Shadowrun's magic system.