Ok, so I've read these rules four times and they still don't seem right. It appears that a Marine waving around a Combat Shield has a 25% chance of ignoring any attack, with a tiny 10% chance of the field overloading. Storm Shields have a 55% chance of doing the same. Surely that cannot be right, since a Marine not bothering to dodge or parry with a Storm Shield has a 55% chance of ignoring all incoming attacks, and those attacks that do land must get through his impressive Armour and TB to do any damage!

Am I missing something?

Yes. The point of the game.

Space Marines are truely awesome (as in awe inspiring, not the watered out modern usage of the word that just means 'cool') and thier weapons and equipment are similarily so. Of course, the number of thier enemies should be equally daunting. If you look up the rules for hordes and some of the more dangerous enemies you'll see that even a storm shield won't wold off the enemy forever.

Take a look at the rules for assault terminators from the wargame and consider that those are watered-down versions for the TT.

Yes , terminator armour and the equipment for it are stupidly powerful - and it's only given out when they'll need that amount of power...

You have to consider a Terminator is something akin to a small battle tank, especially with a Assault Cannon or Heavy Flamer

Terminators are sent into the very thick of the fighting where no other troops are expected to survive. It's one reason they're generally the only SMs who use teleporters.