if pike handles an array of a few thousand of these the use of bignum vs unsigned int type could make a difference.
The tradeoff is with the added complexity of all parts on the C level that handle integers of any sort visible to the language (thousands of places in the code). You don't really want that added burden everywhere for the (relatively) small feature in conserved storage.
Some custom applications that handle tons and tons of numbers in that range and don't want to adapt their logic to use signed integers to get the range right may well benefit from using C, for instance, and unsigned integers, but that does not make it the right decision for a much larger language like Pike.