Fixing flats is a pain... especially if one is on a tight schedule.

On the other hand, with today's "armored" tires, I'm not convinced that simply wearing the rubber thin increases risk of flatting. My theory is that most of the protection comes from the tire casing, and thin rubber has less likelihood of picking up debris.

That is, of course, dependent on the casing being mostly intact.

I haven't been filling holes, although I have a large hole in one tire that I have't decided what to do with (other than not ride the tire). I did fill holes in another tire with super glue once... it seemed to work, but the next time I inspected the tire, I had difficulty discerning super glue from debris, and picked several of them out. In the end, I wasn't convinced filling helped.