I'm with aeronut, my understanding (on the '75 scheme at least) is that you commute a percentage of your pension between your 38 point and the index linking point (or 60 point, not sure if they're the same based on what aero said) then after that it reverts to your original.
Based on that, mad not to, as 20 years worth of inflation/depreciation will affect your hard won cash if you don't commute it (up to the index link point at least), and on top of that you're losing interest you'd be earning/interest you're paying on mortgage unnecessarily.
In addition, it is there to use if you need it (if you don't schwack your house payments with it) and is there for your family if you die earlier than hoped.
On the downside, is there to be blown if you're not careful with it!