I get the feeling that "high spend, few nights" customers might not be in the loyalty game and might actually pay for the benefits anyway, so giving them is basically just reducing the net income you can extract from this customers group. Buy doing it on night, they incentivize people who usually stay at higher-end hotel (where the lounge benefit is important and expensive) to maximize their night count... and since IHG has many places with "cheaper hotels", maybe their strategy is to orient those customers toward lower-end hotels in their group, not to miss on nights, when their regular spending pattern would make them look at higher-end proposition from other chains?

Or maybe they didn't think this through. Also they'd need to prolong the benefits ladder. It's hard to do more than 100 nights in a hotel chain and people exceeding this threshold are probably a handful. People who get more than 180,000 points (roughly 100 nights in points based on Diamond equivalency) are probably orders of magnitude more plentiful.