The weight of your vote depends on the number of bitcoins you control. The minority of people that control most of the bitcoins will dominate. The wallets that have the most bitcoins belong to big miners, exchanges, Coinbase, and a few others. If you have less than a bitcoin in your wallet, your vote isn't worth much.
Regardless, it is better to have the votes of the developers + the miners + the bitcoin owners vs just the developers and miners, don't you think? It is not the case that the bitcoin owners will decide the vote, only it will influence it and demonstrate consensus of those who are most invested in the outcome of the decision; those who hold the bitcoin.