Agree that it doesn't contribute as easily to retirement income as investments or cash, although living mortgage/rent free greatly decreases your cost of living, so maybe instead of 200k in retirement now you only need 100k (plus then your net worth continues to climb with your home's appreciation). Push comes to shove, can relatively easily tap the equity with a HELOC.
While real estate has historically appreciated slower than the stock market, the leveraging any average Joe can achieve in real estate can yield much higher returns. A house purchased with 20% down appreciating at 4% a year is a 20% return on your investment. My primary home appreciated 15% this year which means a 75% return on my down payment. While I certainly don't anticipate that pace continuing, I do think looking at the scarcity of any available homes in my area, lack of new construction, and low mortgage rates the next few years, 6-8% is quite reasonable next few years.
Anyway, I consider real estate to be a healthy and frankly a really enjoyable way to diversify. There's no enjoyment to be gained from ETFs, but I'm on cloud 9 every time I'm at my beach house.