I don't think that you can apply any 'rule' to cats being indoor or outdoor - it all depends on the cats, their circumstances, and yours. However, in an ideal world I believe that the outdoors is a cat's natural environment, and that they are in general happier having outdoor access.
My Meesha and Sabby, in particular, would become depressed and deeply unhappy if they couldn't go outside. They absolutely love it, and literally frolic in the garden when they're let out in the morning! They sometimes go out of earshot, to fields over a side road (often together), but mostly spend their time in the allotments and gardens to the rear of my house. Shanti would probably be quite happy as an indoor cat, but even she enjoys spending most of her time outside, although pretty much confined to mine and my next door neighbours gardens. I have a very busy road to the front of the house, but I'm in a middle terrace so it's a bit of a trek to the main road, and nothing whatsoever to attract my babies there. I do worry, of course I do, but I feel very strongly that to keep them in would be pandering to my feelings rather than theirs. I just put myself in their shoes, and have decided that I'd rather have a riskier, but much happier, life. I feel to keep them in would be tantamount to imprisonment, but I don't think this applies to every cat, and those that never know the outside are probably fine indoors.
If they kept coming in with torn ears, wounds or abcesses and the like, I would reconsider. But they don't appear to fight, even though there are loads of other indoor/outdoor cats in the area. Meesha and Sabby are VERY wary of other humans, except my lovely next door neighbour, so I don't have fears about them being taken, or being caught and hurt. Shanti is also cautious, and doesn't really go anywhere to get into trouble! Added to all that, I've met most of my neighbours (about 30 households) and if they don't have cats of their own they are cat-friendly, and know every cat by name in the neighbourhood - and where they live. I know if one of my babes got into trouble my neighbours would help them, and let me know immediately.
I do believe in keeping them in at night, though, as there's a lot of foxes in Bristol, and Meesha in particular is really quite tiny.