This implication is flat wrong in 3.x. It may have been a bit accurate before 3.0 made defense a stat that actually means something, finally, but all of the research on Foresight has proven this to be an inaccurate assessment of how effective Foresight is for physical mitigation, now.
My Paladin currently has 2524 Defense with Protect and 3029 with Foresight on top.
According to calculations done in early 3.0, hitting this cooldown is ~15.5% physical damage mitigation. I would say that's actually significant. It's certainly weaker than Rampart, but it's a lot closer than you're implying.
Comparison of different physical tank buster mitigations:
ShO+Sen+Shel = .8*.6*.77 = ~36.9% damage taken
ShO+Ramp+Shel = .8*.8*.77 = ~49.3% damage taken
ShO+Fore+Shel = .8*.845*.77 = ~52.1% damage taken
ShO+Shel = .8*.77 = ~61.6% damage taken
So yeah, it's clearly not better than Rampart, but it's absolutely not worthless. Implying it's "not a real CD" is silly. Would you say Dark Mind is "not a real CD" without Dark Arts? It's essentially the Magical equivalent to Foresight at current gear levels.
If you're eating physical tank busters, Foresight should -absolutely- be part of your CD rotation. If it isn't, you're doing it wrong.