The investors will be sure to keep you tied in, and that money can massively derisk the project for you. I encourage every founder I speak to do this.
Non founding executives with short tenure doing the same is a little shadier, but everything is a negotiation and nobody was forced into the transaction. A rank and file employee can push for whatever they like - more money, stock or even liquidity if they have enough perceived importance.
Look at it from the other side. Say an exec joins a series C business and plays a big role in growing the company to 3x revenue in 2 years. At the next raise they ask to cash out some equity to buy their beachfront house. Seems a fair enough ask, and with that track record it would probably be worth investors writing the cheque.