As much as I agree with you the same thing is often said about weather apps. However the really accurate ones (e.g. Dark Sky) that can predict to the minute and give intensity are really useful. For example I was able to plan my lunch break using it yesterday so I was out of the office in the 30 mins of dry weather between rain showers. The the UV sensor you could probably work out what strength suncream you need and if people apply the correct strength maybe in the long term skin cancer rates will fall. Small change in behaviour due to better information, big improvement for society.

In an area that reaches extreme UV indexes I would find it useful to know for a for a given start time in the morning and the afternoon how much time I have before burning for the conditions on the day.

UV forecast graphs take some of the guesswork out of location and time, but I find that it hard to judge how much cloud cover is needed to drop from a high to a low index range. Sunscreen is great but it's not much fun to have to be covered in goop every day. Getting burnt when it's cloudy isn't much fun either. Knowing your exposure would limit the amount of cream.

Maybe. If they were already including a photocell, it might have been more or less free to use one that enabled this feature.

UV levels can also be counter-intuitive, where the type/amount of cloud plays into how much is blocked, or where a partly cloudy sky actually increases UV levels. Or where, using my dumb experience as an example, you forget to consider that the spring sun in Florida is more powerful than the summer sun in the north.

Ask all the tourists who come to Aus/NZ, go outside for a whole day and fry to a crisp. UV levels aren't necessarily related to sunlight.