If you fly out of small airports and only taxi 3-5 minutes (3 minutes is CFM warmup time) then these have no use and you are carrying around extra weight. If you are flying out of busy airports and your taxi time is 15-45+ minutes these will save enormous amounts of fuel, especially on 737 and A320 sized and smaller aircraft that fly a lot of legs. Great idea.
Who is monitoring your engine start? At a busy airport, the 7 aircraft behind you. They will be eating your smoke if you have a tailpipe fire.