While the monthly savings is nice, the biggest advantage is the faster pay down on principal on a lower interest mortgage.
If you really do plan to stay for 30 years, you can probably chain refi 2-3 (or 20 if you are ps9) 5/1 ARMs (or 2 7/1 ARMs) for the first 10-15 years and then refi into a 15 for the last 15 years to give you better cash flow than a 30-year fixed. It's a hedge against whatever the rates are because the ARMs and finally the 15 will always be lower than the prevailing fixed 30-year rate.
But that is theoretical and the 10th man could end up blowing up in your face. Also, in that scenario, you will always have to qualify for those refis and who knows what your finances will look like then.
It's really depends on each person's financial situation. For us, I don't imagine a drop in income in the next 10-20 years nor do I expect a huge jump in rates within that span (although no one thought rates would drop to 3-4% either), and I doubt we'll become landlords again so we can risk the ARM.
For you, just like solar, you have to do the math (and depending on your loan amount, an ARM can save $200+/month over a fixed).