In theory, the attic temperatures are not an issue of concern. The roof ability to vent the moisture is what is the critical element. Yes, zoned AC systems that are in the attic do have to work harder if that is the case, but speaking purely theoretically, the temp is not the issue if your envelope (air) and insulation layer of your attic floor are intact and sufficient. You will always want to have more incoming air than you have exhaust air. If not, you run the risk of creating a negative pressure in the attic and thereby removing conditioned air from the conditioned zone below. This is what is often happening when people run attic fans and feel like they are "working" by keeping the attic cooler. They are, but at the expense of sucking conditioned air into the attic. I am guessing that you have soffit vents for the incoming air? Have you checked them and are they unblocked and open? If they are and you have good continuous make up air coming in, putting a couple of vents in the roof (not powered) is okay if you are concerned. Again, it is not really a necessity if your attic insulation and envelope air tightness is good. If you are worried about a second HVAC system running in inhospitable environments, a little more exhaust to get the air moving shouldn't be too back. That being said, even a small ridge vent should flow plenty of air. Check your soffits.

A properly installed ridge vent---on the style of roof that is recommended(gable end roof) with sufficient open soffit vents(An absolute necessity) will ventilate an attic space naturally for no cost other than the cost of installation. Ridge vents vent the hot air in summer or winter at the highest point of the roof---the peak by utilizing the fact that hot air rises---and rising air creates a low pressure area under it----where the soffit vents are---pulling in cooler air to replace the escaping hot air. The only way to stop this from happening is to physically block either the ridge or soffit vents. The only down side to having a ridge vent installed is it does change(slightly) the profile of the ridge of the roof. A properly built ridge vent will not allow rain/snow/insects into the attic.

Of course, this is just speculation on my part but it appears as if the roofer that did the job installed the ridge vent as a dummy ridge vent and didn't intend to have it functioning. I say this because when the owner of the company came here, he expected to just go in the attic and cut away the underlayment. He came back down from the attic after a while and said that he couldn't cut it from inside the attic, it was just too hot up there and he couldn't get to it well enough. Well....I went up there and there are no obstructions keeping him from getting in there and cutting out that plastic...BUT what I did find is that he had cut into the plastic so I looked inside the hole....what I saw was not what I had expected. What I saw was the backside of shingles with only a small opening of about an inch. So not only was the plastic there, the shingles were not installed in a way that you would install them to put on that ridge vent. So he is going to have to take it all off and actually saw out the shingles to make a hole for the air to escape. All the while he kept saying that it isn't even required for there to be a ridge vent over this garage. So I don't know if they were being deceptive or if it really was an oversight. I guess only god knows the answer to that one...HA! But I do know what it looks like. Also, I doubt the company owner knew this because he wouldn't have cut the plastic opened had he known because he had fully intended to just cut it opened.

I'm still not clear what you're planning to do. Are you putting the insulation on the floor of the attic, or under the deck of the roof, covering the entire underside of the roof? If on the floor of the attic, you need some kind of baffles from the soffit opening up to and a bit beyond the top of the insulation, so that it doesn't get into the space and block the air flow. If you're putting insulation on the underside of the roof, you'll need baffles all the way from the bottom to the ridge vent so that there's a free flow of air under the insulation from the soffit to the roof. Here's a video that's of value to illustrate first putting baffles on the underside of the roof, before adding insulation on the underside of the roof.