I'm going to add to the chorus, but "it depends on your audience". If you feel you have matched turbo and real miles, and you want to compare your output to last year, then count them. If you want to make sure you cover a certain "mileage" each week, and need to combine indoor and outdoor miles into a single figure, then again, count them. If you are working with a coach or team that has targets, then count them.
If you are in a challenge that bans turbos, then don't.
The first year I went further than 8000km (5000miles) I shared that on facebook to my mostly non-cyclist friends. It seemed valid to share that as it's a meaningful, understandable number. Some friends mentioned it was further than they had driven (it was certainly further than I drove that year). I really wouldn't share on facebook, that I used a piece of gym equipment that is calibrated to be accurately equivalent to road cycling to travel the equivalent of x miles. That seems as arbitrary as multiplying weight x reps x sets and saying I lifted 1 tonne in the gym today. Might be true, but it's just a number.
If you use other pieces of stationery equipment that measure wattage, do you take that number and convert it to miles on a bike?