The vesting interest doesn't do nothing. It allocates more of the costs of 'real' inflation (i.e. the other categories) to non-powered-up investors. We could agree or disagree on whether that it is a good thing but that is its actual effect not 'nothing'.

I'm not in favor using vesting interest because I don't agree that it is even in the same category as the rest of inflation. It isn't real inflation at all, it is a cost allocation between investor classes. Shifting it to non-investors is a real inflation increase.

Content rewards are a cost that investors (unequally by class; see above) pay in order to attempt to add value to Steem by making payouts to users with votes (some of which in fact represents the same sort of projects making posts to appeal for funding/votes that SPS itself addresses). It is entirely reasonable for investors to say that they want to start making some portion of those payouts to users via slightly different voting and payout rules (optimized specifically toward targeted and/or ongoing work and projects than the existing pool, even though the existing pool is often used for that too). That is what SPS is actually doing.