Yes, if your app broadcasts the op, on your server, that’s a solution. But if a user broadcasts it from their browser, it starts to get tricky. Does your app then send the block number to you? Did they even use your app? Or did they use SteemConnect/Steem Keychain? Did they use a third party app that doesn’t know it needs to report the block number to you?

In a sense, the blockchain is the method of communication, so it’s redundant to also use another protocol to report the block number. Ideally, you just monitor the blockchain for new data or just some other indexing strategy, not both.