DNS Round-robin failover and load balancing

1. because it's cheaper and easier than anycasting (BGP) and because it can be implemented entirely on your own hardware
2. from a load-balancing standpoint, there's very little you can do about the browser caching DNS results in contradiction to your TTL settings.
3. I'm not sure why ISPs would rewrite TTLs, Most requests will be from browsers, so I'm not sure it would be cost effective for them to invest in the infrastructure to do so.

I use DNS (GSLB actually) over anycasting for a couple of reasons. We've discussed BGP several times over the years, and I choose DNS over BGP because it's easier to do and, in most cases, just as or more reliable. Multiple facilities, clustered DNS servers, combined with proactively monitored GSLB and a TLD hosting service covers most potential faults.