Those not running big tax bills or businesses usually don't spend enough to generate the same amount of points as the sign on bonuses. In my case, I would typically spend about $2k-$3k per month on Amex including across the supps, perhaps more when 'opportunities' like the recent Myer offers come along. Thus it takes several years to earn the amount as a decent sign on bonus. It's a no brainer.
This article from today seems to suggest that a $60k per annum spend is a tipping point for some. Not in my book but the analysis is pretty superficial.
The other benefits, particularly since the big deval, are more tangible and obviously relative/subjective. With the constant "enhancements" by airlines, even churning is becoming less lucrative.
I'm about to ditch my last AmEx and will rely on the wife's for any worthwhile statement credit offers, although we won't be able to magnify the gains as if we both had Amex with several supps. This appears to be as close to having one's cake and eating it as possible.