It is not the same as contributing proportional to stake at all. There are accounts that NEVER post or comment, that don't even curate at all, but only flag. There are accounts that do NONE of those things, but funding SPS on rewards will increase the benefits to those accounts and increase their power relative to all those accounts that effect the creation of value, because funding SPS through rewards will decrease the incentive to effect the creation of value, but it will not be paid for by accounts that don't create value.
Why do you want to give such stake a free ride on the coattails of those creating value?
Fund SPS via stake directly, as the benefits of SPS will inure directly to stake, not rewards. Doing so increases the incentive to invest in Steem by providing funding clarity and certainty, while not draining the beneficial impact of rewards generating activity that creates the value of Steem.