It is useful to have tray indoors but my cat doesn't use his. He always goes out, never do his business indoors. Our litter tray is just taking up space, he has no interest in it.
My boyfriends parents live on the countryside, and they always remove their litter trays once kittens are grown up, they've had around 10 cats over approx 20 years, all been in perfect health and lived for a very long time. They have still been able see the signs of illness/infection and take their cats to the vets - and they don't think it is neccessary to have litter trays 24/7 unless cat is unwell and needs to stay indoors.