The more people who run a full node, the greater the decentralization[1][2].

Using the chain as data storage, rather than currency, costs everybody, because it increases the rate at which people are discouraged from running full nodes.  It increases the costs of that dataset that cannot be pruned, and must be carried for eternity: the unspent transaction output set (UTXO), the list of coins available for spending.

Right now, it remains within the realm of a hobbyist to run a full node, especially with the recent memory usage improvements in bitcoind.  But one day, that will not be the case.

By pushing back on data spam, we reduce the rate-of-increase on blockchain resource costs, and reduce the disincentive to run a full node.  We push back the day at which there are just a handful of archive nodes with a copy of the full block chain.