They will join because its faster and they save money. To use xRapid software means they are using XRP. Banks only hold XRP for 3 seconds. They don't hold it like investors HODL, they can but that's not what XRP was designed for. A bank buys $140M worth of XRP, sends it to a bank in Mexico where it's sold for pesos and both banks are happy. XRP is the bridge asset, not the asset they are holding. Bitcoin and litcoin could be moved around in the same way as well. All that is required is the trading pair with XRP and enough liquidity.
No it's not a stock, you're right, they don't claim to be. In order for cross boarder settlements to take 3 seconds and cost less than a penny XRP needs liquidity. To achieve that there needs to be people buying and selling. It's a speculative asset based on the value people see XRP providing to Ripples customers. Hope this makes more sense.