The economics are very simple. Single engine taxiing saves money. The more flights you do, the more you save. If you have a start problem then that is tough but really, ho many start problems have you had? My company have been doing it for near enough 12 years without a problem (100,000 flights per year). I'll also suggest that as soon as the economic case for having electric taxi motors is clear, airlines will fall over themselves to get them installed. Complexity is someone else's problem, looking after cash is theirs.
PM