There are a lot of details about this point...
Police have a bad habit of harassing populations (several riots in France were a result of policing practices such as asking everyone for an ID). It helps to contribute to send a message of unwelcomeness to people who are likely already disillusioned in the system.
It's unlikely that more police could stop attacks. Maybe deal with the aftermath/manhunts better. But it sounds like more precision operations (including human intelligence) will go a lot further.
That's exactly what more police will do. Make people feel safer, not actually be more safe. It's security theater. [1]
That's simply not true. A larger police force translates to more comprehensive law enforcement. If it doesn't, then there's a systemic failure in the way police are run (and we have plenty of examples of those too). Of course "more police" isn't the only way to increase the effectiveness of policing, but it does help.
The security theater was specifically about TSA and other untrained (or minimally trained) symbolic gestures. Actual policing is not theater.
More police can help with theft, possibly with solving murders. It will do almost nothing to prevent terrorist attacks. This airport attack is a prime example of the reason why. You can put large checkpoints everywhere, but there will always be, by definition, a line of people that haven't yet passed the checkpoint and this is where they will attack.
Belgium recently said that nearly every one of their available officers was running down ISIS / jihad leads, they've essentially stopped investigating other crimes and still don't have the manpower required. They're in desperate need of more well-trained police.
More & better trained detective-type police can be useful, but the vast majority of any police force isn't doing detective work.
It's expensive. It has some effect on civil liberties.
Does it work? The cost and human rights interferences are easier to take if more police works, but there's no evidence that it would.
Why is this a bad thing? I feel safer with more police presence but maybe that's just me.