Ken,
Your observation on the annuity-like benefit of paying off a mortgage is right on. Basically, paying off a mortgage gives you a life-time pass on debt service but, unlike an annuity, you still have the money in the form of increased home equity. So I would pay off any mortgage debt well before buying a life annuity.
Your deferral of Social Security benefits amounts to a major purchase of a life annuity. The longer you defer, the greater the benefit increase and Social Security is an inflation-adjusted joint and survivor life annuity. Since you will be spending some of your IRA or other savings to support your household, the period from now to age 70 is one where you are, in effect, exchange volatile IRA assets for safer life annuity assets.
The studies I have seen indicate the trading investment assets for a lifetime annuity DOES dampen the upside but it also dampens the downside and increases the odds that your lifetime income will be sustained. The only question is the lottery ticket of our estate size. In your case, with a 95 percent chance of achieving your goals without buying a life annuity, it's hard to make a case for needing one. If your chance was only, say, 70 percent, however, it might be a really good idea to consider using a life annuity. That, by the way, was the basic issue Price Waterhouse was trying to address in their recent study. You can read the column on it at this link and download the study itself, "Retirement Vulnerability of New Retirees" from a link at the bottom of the column.
http://assetbuilder.com/blogs/scott_burns/archive/2008/08/15/will-you-die-broke-maybe-not.aspx
Scott