Compared to being required to delegate massive amounts of authority to a TTP just to log in to a service that uses non of that authority, it is a simple low impact way to log in. A generic TTP-free signature based login would be great, an I'm not sure, but you could probably just use your memo key for that, but as far as TTP versus micro-transaction goes, micro transactions should be the preferred log-in only option IMO.

A 4th possibility with a TTP that I feel could actually work is a TTP that sells client certificates using memo field in the sell transaction the same way that micro transaction login would.