So you have to stop somewhere approximately 2 minutes taxi time from the runway to start your engines. Then when one doesn't start eco taxi back to the ramp to get maintenance to have a look before having another go. Meanwhile everyone else in the queue is fighting over when to stop and start engines.
For some overcrowded hubs at rush hour this may be ok but can you imagine being in the 40 minute taxi queue at JFK only to have to taxi back in with a problem?
For all the other times when your taxi is less than 10 minutes this just will never pay for itself. A ground based solution is a much better idea as it adds no extra weight.
But when you're driven by a tech company trying to sell stuff and confused by illogical "green" arguments I guess you'll give anything a go.