REITs are one of many equity-like assets that sharply correlate in times like 2008 when there are crashes. There is no need to tilt your equity allocation toward REITs.
Small caps, Value stocks, international stocks, emerging markets, etc. all were strongly correlated during the crash. Does that mean that you shouldn't buy any of those assets? I suppose, using your specific rationale, that means you shouldn't tilt to any of those areas. That's a reasonable suggestion (and the three-fund portfolio is fine), but there are also reasonable arguments to hold/tilt to various subclasses of assets.
As you suggest, it's not prudent to believe that diversifying among equity-like assets will provide a safe haven during a crash...but that's what bonds are for. That doesn't mean that diversifying your risky assets is undesirable.
Retirement investing is a marathon.