[Bruno]

To the best of my knowledge Blocking spells can be cast on other people. GURPS doesn't really have "self only" spells, with the exception of Recover Energy - but Recover Energy isn't cast anyways. It's sort of a weird advantage.

Nothing says that only Regular spells can be cast on others. The other class of spells with restricted targets is Enchantments, but they're about as far from Blocking spells as you can get :)

We have always played that you cannot use a Blocking spell outside of an actual defense situation, because that's sort of cheeky. Were Magic being rewritten, I would suggest simply ditching them and allowing a "power dodge" from the base spell (eg. Missile Deflection) but since that's not happening any time soon, I'll just have to mutter about it on the forum here.

The Blocking spell has the advantage of you not having to have thought of it in advance; you can't anticipate every problem. Also, remember, every spell on gives you a cumulative -1 to cast all other spells. If you put Armor on all five party members, you are now at -5 to cast all other spells. You're at -5 to cast Fireball. You're at -5 to cast Minor Healing. It just generally sucks.
Putting Armor on one or two party members you judge it to be most necessary on, and relying on a Blocking spell for emergencies avoids that.