Ventilation is a two part process. First there has to be a vent out to get rid of the air. However for that to work there also has to be vents letting air in.
Another very big factor is the location of the vents. If the locations short circuit or leave dead areas, those areas will not be properly vented. The shape of your roof and attic area in relation to the location of the inlets (lower located vents) and exhaust (high vents) determine this.
I suspect that since the roofer suggested you need some venting on the front of your roof, that there may be a dead area situation and just adding a power vent will not change that. At best a power vent may increase the flow so you can get buy with lest total vent area. It also could create a short circuit and actually reduce the effectiveness of your vent system (although I would believe this is the exception, not the rule.
In short, I suggest having someone who knows what they are doing take a look at your situation and make a second recommendation so you can confirmed the first of put it in doubt.