Most of them don't have proper firearms training and hence are more likely to be a danger to themselves and the public if armed.
The Met Police firearms unit, who are trained, generally seem to hit whatever they're shooting at.
Unfortunately what they're shooting at is not always what they're supposed to be shooting at and they were already subject to various court actions even prior to shooting Mr Menezes for shooting people who weren't armed, were the wrong people and so on.
So heaven knows what the untrained ones would be like when let loose with firearms. I'd prefer to avoid finding out.
I don't have any expertise on this subject, so say this with some diffidence, but -
on the face of it seems common sense that a police force in which the only armed officers are those who have volunteered, been vetted and intensively trained for the purpose is likely to be be more effective and safer in the use of those arms than a force where every officer carries a gun as a matter of course, whatever his competence. I'm not suggesting this is an argument against routinely arming police, but simply that when guns are actually used the average level of competence is likely to differ.