17 million mile man is Randy Petersen - virtually all of his travel presently is "work related" so the miles that he earns are paid for by companies or at least tax deductible if its paid for by his company. So he's earning miles that are at least subsidized by others. Miles "saved" for personal use upon retirement or other personal travel.

I would generally agree that "hoarding" miles is generally not a good idea if you're personally paying for tickets since miles never really increase in value - but often are devalued or harder to use. It can be a balancing act though, if you're trying to qualify / requalify for status (which gives you quite a few perks - and even more miles) and using miles. 99% of our travel is personal (and paid for by us) but in the quest to requalify for at least Delta Gold status each year, we really have to make an effort to keep our Delta mileage balance (between 2 of us) below 1 million.