Look at it this way: say I have an Athletics of 5 and a Discipline of 5. I can, without too much difficulty (other than perhaps a Fate point tossed into the pot) cast a spell that gives me a Block of 6 (especially if I have a bonus from a Focus).
Now, there's a pretty good chance that my defense roll will beat my Block of 6, though my average roll will be slightly less, at 5. I could get lucky and roll up to 9 on my defense roll (if I roll ++++), which would make the Block irrelevant. But here's the important part: my defense roll could be as low as 1 (if I roll ----). Without the Block up, I'm probably in trouble. With the Block up, I can claim the Block in place of the roll.
As a more 'normal' example, let's say I have those same stats as above, and I get hit by a rifle-wielding thug who manages to roll a 5. With a perfectly average roll (0000), the attack matches my roll, generating no shifts, but the W:3 adds to this and I take 3 stress. If I had the Block up -- even though it's only 1 higher than my defense roll -- the attack would miss completely!
So yes, if you can count on being lucky most of the time (or are willing and able to dump Fate into rerolls to make your own luck), then the Block isn't of much use. I dunno about you, but I'd rather not rely on my (questionable) luck...