For me, it comes down to a very simple realization:
In 5, 10, and certainly 20 years... what will I remember more, what will leave the most lasting impression? The extra $1500 in my pocket from selling my 100K AmEx offer, or flying Singapore Suites with my eishes chayil on a trip to New Zealand?
Res ipsa loquitur.
versus:
I 100% agree with you, but it's not only $1500.
You've probably earned somewhere between $60,000 - $100,000 in miles the past couple years (CMIIAW).
So yes, you will remember these past couple years and the unbelievable trips you took, but maybe that money would go to for a down payment on a house or investment that will last more then a couple years.
Of course it depends on your financial situation.
PS. I use my miles for flights.