First of all, isn't Neiman Marcus a store??? Well, stores sell a variety of products (most of which they do not produce), over a tremendous range of prices (ruling out the possibility of awards based on number of visits during which you spend SOMETHING), and so rewards based on spending are about their ONLY option. So they didn't choose it because it was the option they liked the best, they chose because it was about the only option avialable. (The only logical alternatives still related to $ spent, but computer in different ways, using thresholds or no thresholds, aggregate or per visit, etc.)

On the other hand, industries which create their own products (airlines sell seats on flights, hotels sell rooms for nights, movie theaters sell admission tickets to a performance) have a variety of ways of rewarding to choose from, including simple ones like per transaction (that's how most movie theater chains do it, that's how some hotel chains do bonuses at least calling it "per stay", and that's close to how Southwest does its FFP because it hands out its "credits" based on flights, not miles) to more complex ones like miles.

You want a downside: It'd be taxable! The only reason that efforts to tax miles have failed is because no one can come up with a FIXED valuation for miles. Miles vary in value drastically both in how much it costs to earn them (cheap when done filling out surveys or applying for annual-fee-waived credit cards, expensive when earned flying on paid international first class tickets), so the IRS has generally given up on being able to come up with any basis on which to assess a taxable value. But it miles were nothing more than a "cashback" (which is all any points/$ essentially equates to), they'd have a very defined value and if they stayed as popular as now (which I have my doubts) you'd see renewed efforts to tax them, this time successful ones.

Furthermore, it'd make the math "too simple". A lot of people right now don't "do the math" (or do it incorrectly) and thus go out of their way to earn miles while spending way more than they should have (had they actually thought about how much those miles are worth). A lot of airlines make a lot of money off of this. Similarly, the airlines take the most popular awards ("saver" type domestic coach awards) and "price" them to be the lowest value redemption possible, but that doesn't keep most people out there from using them only for that (you can get a "free flight" for "just" 25000 miles!). Again, the airlines make money this way, because these seats are restricted and they only give them away when they feel they'd not likely sell them.

If the math were simplified, value-hungry FTers would no longer find much value in miles, while the airline would less value in them also (because they could use them to trick consumers into "wasteful" spending as easily).

And getting back to stores (tho not necessarily Neiman Marcus specifically), the other game that some stores like even more than loyalty programs is rebate programs, again because they can be used to trick consumers (who see the savings they'll get from the rebate, but then will forget to send the rebate in time or otherwise not qualify) into spending more than they otherwise might.

Finally, if miles were so simple there were no giant "loopholes" to exploit, where would FT and IF be? (You might not be asking this question here because this website might never have come to exist!)