Another big problem is keeping the training up. I’m in the TA (no laughing at the back) and I shoot with the Army a fair bit, quite a lot by normal TA standards actually, and I know from experience that people that do the minimum training for their qualification are _usually_ not that competent in their weapons handling*.
If all police were armed and only had to qualify once a year, which is possible, then I would seriously worry at the level of skill that would be developed and maintained within the rank and file, especially given the lack of budget for training and practice that I am pretty certain would be enforced.
It would be worse with pistols too. Short weapons are a lot more dangerous to the people that are around you than rifles are and, whilst I can see an argument for allowing expansion of less lethal weapons within the police, I would not want to see expansion of the use of pistols for the same reasons as above.
If a specialist skillset like ARU is kept to a small, highly trained and maintained group, then they will be safer and better able to carry out their duties.
* – some people can keep up their skills between qualifications, but they will _not_ be as good as someone that does things regularly and often.