It happens when you have a power gamer GM who sits and makes characters for fun, to ensure that combat balance is maintained.
I always make at least three or four sample characters whenever I play a new system, to test the extremes. It very quickly became clear to me, that as the system is now Brawn/Soak is the single most powerful combat statistic in the game by a wide margin for players.
I am not mathematically inclined, but I have a reasonable instinct for numbers, and Brawn/Soak is a statistic that becomes more effective as its value increases. Although you are right that no one is completely immune from a really, REALLY lucky shot, damage in this system is still fairly static. Even at the extreme edge of competence, a weapon will rarely do +5 more then its listed damage. This means that edging your soak value higher will make you all but invulnerable to an ever widening range of damage sources. Even if the hit is particularly good, the amount actually getting through is limited and could possibly be considered trivial.
At lower soak values, this isn't obvious. At soak 3-6, most weapon hits are still serious, and a blaster rifle is a real danger. At soak value 8-10, the heavy blaster pistol becomes a paper cut, and the blaster rifle far less intimidating. This is a concern, as I believe these to be two of the most common mook weapons you're likely to see. At 11-15, heavy blaster rifles and even light repeaters become far less dangerous.
In essence, this means that a character stacking Soak and Brawn can wade through minions with almost no fear of serious injury. Enemies armed with weapons that will leave the rest of his party a crispy ruin will bounce off of him. If the GM upgrades to compensate, the remaining PCs will become even more trivialized during combat encounters as the tank is the only one with even a chance of surviving on a battlefield litttered with repeating blasters, vehicle weaponry, missiles and other heavy weaponry.
On top of that, it makes increasing your Wound Threshold even more valuable. Purchasing a talent that gives +2 WT is no longer a small upgrade, but a whole additional hit that you can take. The difference is fairly dramatic, when you compare the Soak stacker to any other combat build, at least in my opinion. In MMOs I saw the concept first coined as Effective Health. And that's what stacking soak does: Dramatically increases your effective health.
A few numbers of what's theoretically possible:
Starting character (Droid, Marauder): Soak 8 (other species would have soak 7, still very strong). At this level, a starting character is all but immune to blaster pistols, and if we assume a blaster rifle will do between 10-13 damage on average, he will be taking 2-5 damage per hit. While that's significant, compared to his soak 3-5 team mates the difference is huge.
200ish XP, mostly spent on Marauder talents: 2 more ranks of Enduring (+2 soak), Dedication to get Brawn 6. At this point, the soak value rises to 11. Possibly more if better armor is acquired, or Cybernetics is used to bring Brawn to 7. At soak 11, the blaster rifle becomes a paper cut. With cybernetics and modded armor, the character is at Soak 13, trivializing the blaster rifle almost completely and making the Light Repeater much less of a threat. At this point, the character is essentially mook-proof, especially compared to his team mates whose soak values have not moved much from their original point. The tank will also have a large Wound Threshold, made even larger by his Soak value.
At around 400 xp, the character has branched into Bounty Hunter for Gadgeteer goodies. Further increasing the soak and defense on his armor. In Heavy Battle Armor, he has a soak value of 14 with a Defense Value of 2. If he sticks a personal shield generator on that, its Defense 4. This is just insult to injury though, as he's already a walking tank for all intents and purposes. At extreme amounts of XP, he could raise his total soak to 15 with another Enduring point from Bodyguard and purchase things like Dodge.
Now, this is only a problem because of two things: 1.) Weapon damage does not scale overmuch. PCs will see an increase in damage, but mooks and NPCs for the most part won't. 2.) The rest of the PC group will NOT have such a dramatic increase in survivability. The difference is simply too large, in my opinion.
I already posted about this on the beta forums in the mechanics section, but no one really seemed to respond or care.