Protecting against what, exactly?
I think Hairy Muppet makes the most pertinent point - if you look at the small print of the CE certification details for back protectors, all this really means is that they will offer some limited protection against soft tissue damage, that is bruising.
That is a good thing, of course, but I think there's loads of people out there who have bought back protectors on the assumption that they will save them from a broken back.
While there may be some very marginal occasions when this might be true, my understanding is that the vast majority of serious spinal injuries suffered by riders involve mechanical actions that a back protector can do nothing whatsoever to protect against.
For instance, if your lower body hits a solid object at speed, your upper body will continue going forward, causing your spine to hinge and massively overextend beyond its normal safe range of movement. The energy involved can easily be enough to crush vertebra and this is when the irreversible damage that we all dread can be done to the spinal cord.
I think makers of back protectors should be clearer that you're not buying protection from a broken back.
The same thing could be applied to helmets up to a point. We tend to have an optimistic view of their performance, not least because they have to pass official tests. I know I think of my helmet as offering significant protection in pretty much all circumstances. But you don't have to be travelling very fast for its value to be near worthless, if, for, instance you're unlucky enough to crash into something solid.
I suppose the point I'm making is all safety gear is potentially useful, but should never be thought of as an equivalent or substitute for the kind of heavy-duty safety measures that car drivers, say, enjoy, like steel cages and seat belts.
Perhaps the most important thing of all to invest in are the riding skills that will lessen the risk of you ever having to test out the efficacy of your chosen safety gear - observation, defensive riding, machine control, etc