Because it is my understanding that turbo's wreck normal tyres? So I don't fancy changing my tyre and my g/f tyre each time we want to go from turbo to outside.
I've done hundreds of miles on my trainer and I see no adverse tyre wear. Use the calibration in the trainer software to make sure my tyre is nicely contacted - not too firm against the trainer wheel and not too light on the trainer wheel.
One tip - never ever apply the rear brake with the trainer spinning, otherwise it'll eat your rear tyre pretty quick.