As to why I want to do this, I'm not sure I do, but I foresee several scenarios in the next few years where I might want to have a chunk of cash up front and pay it back quickly -- but I want to avoid taking a loan against my house or using up cash on hand.
My situation is that our tax rate will definitely not go down when we retire and might, in fact, go up as minimum distributions are factored in. As such, I don't mind having a portion of tax-deferred assets in a cash or cash-like investment, which could then be used for loan purposes. What I want most to grow are my Roth assets and my non-tax-deferred assets.
I suppose a HELOC would serve many of the same purposes, but I really prefer not to borrow against the house. It may be a comfort-level vs. what makes the most sense.