A 30ma RCD will tripp if you touch a hot wire even if eart is not connected
That's true, but surely you're not advocating to leave the ground floating?
With the ground connected, the RCD will trip first on any fault and the operator will spared that 30mA buzz. Under just the wrong circumstances, a 30mA buzz can be directly lethal, and it can always cause a secondary hazard - like falling off stairs, or jerking into a spinning lathe/mill or the like.
Also, the general rule with life-and-limb safety is that you want multiple safeties. With an RCD, a breaker and a ground connection, you effectively have two operator safeties.
With no ground, you'd better hope that RCD is good because it's the only safety the operator has. Incidentally that's what the little button on the front of it is for, testing it against e.g. an open sense coil, or a welded contact or the like.
The breaker is not there to protect the operator, it's to protect the wiring against becoming a fire hazard on overload.
Here in North America the ground is commonly the only operator safety, as you don't typically have RCD protection on anything but household bathroom or outdoor outlets (but I digress). This is true at least with residential wiring, I don't know how it works in an industrial setting.