doubling after 20 years is approx. 5% growth per year (locked in).

It's not that great an investment, but it does make sense to use it as a portion of your portfolio for retirement. I would imagine you'd put around 5-10% of your total networth in this form - may be even stagger it over time (e.g., put in 1% every year starting from 40). This can exist in lieu of bonds in your portfolio - though i would perhaps split it 50/50 with your bond allocation (which gives you some liquidity early if you truly need it).

I would want to have more invested in equities as their potential for growth is much higher, and over the long term, ought to beat inflation (as long as you purchase a market weighted index).