GMK, I think the allowance for loans stems from a (perceived?) participation issue, rather than a benefit at retirement issue.
Offer the opportunity to have at least some access to the money and more people can be induced to participate.
Good for the employees--they are saving for retirement
Good for management--more rank & file participating, the better for allowable HCE deferral rates
Good for asset providers--more assets under management, income from fees (though that offsets the AUM)
Good for the brokers/agents--more commissions
Good for the TPA--generate fees for processing and administering loans
Good for the economy--proceeds from loans going to either pay off debt or purchase goods and services
Good for CE providers--yet another webinar they can charge for
See a downside yet? I don't.