This is an old thread, but what the heck, someone else might read this.
The principal portion of your mortgage payment cannot be considered an expense. If that were the case, the outflows of renting and owning would be on par with each other. We know that they are not.
But is it savings? Yes, but a special kind of savings. Kind of like an egg that costs money to crack open. If you are going to sell your home and cash in on the equity, it will cost you realtor fees, listing fees, moving fees, transfer and recordation tax, etc. So we can't just view this as money transferred into a savings account that can easily be withdrawn. But it is money saved. And it's fun to watch your net worth go up as your mortgage principal balance goes down.
Here's what my balance sheet/ net worth calc looks like: housing costs consist of interest on mortgage payments, property tax, insurance, and budgeted home maintenance and repairs. If you were looking at a rent vs. buy scenario, this figure is what you would compare to the cost of renting. The principal portion of my monthly payment is considered savings at a rate roughly equal to inflation. Home values go up and down in different markets, but on average, they track inflation - so this works. But when looking at your net worth, just keep in mind that to access that home equity, it will cost you something to "crack open the egg".