I typically keep a couple hundred thousand banked in case something fun comes up (like the DFW/SYD in J I booked today for the Thanksgiving holiday week for 125k miles) but I'll often end up giving them away to help family and friends. It also lets me bring friends along on trips that they might not otherwise be able to afford -- good for both of us!

Really, you'd be hard pressed to come up with an "emergency" that required hundreds of thousands of miles so why bank them?