Most people are stating that the problem with easy marine access to vents is that marines have a combat advantage inside of them. While that's true, it's not something that can't be overcome with the skulk walljump vent glitch that lets you propel yourself forward at 9001 miles per hour (convert to Khaara units as appropriate).
The real problem is that it lets marines move very quickly between areas they are not meant to traverse so rapidly, and in relative safety. The route from Skylights -> Overlook and Topo -> C-12 on Veil illustrate this perfectly. Both vents are very easy to enter, and cut off a significant amount of travel time to get from one location to another. You can beat a spinting marine running the normal route going through both of these vents- and not only do you get there faster, you are going to do so silently, and you will be able to win most of your fights. Compare this to the marine sprinting down the hall, who is noisy, easily ambushed, and will not have the draw on any alien he encounters- there's no reason not to go vents.
This is especially bad for the aliens because the whole point of having vent systems in the first place is to give them a mobility advantage in the map to help counter things like phase gates and beacons. When the marines can use the vents as well as the aliens and win more of their fights within, the vent system ceases to become an advantage.
What's wrong with Marines being able to go into the vents? Anyone remember the movie Aliens? :) Solution: Fixed turning speed for Marines when crouched and more crooks and turns and alternative routes in the vents.
Zero7 and Swiftspear, you seem to have forgotten my point. While yes an alien should not feel invincible inside of a vent, they should feel a bit safer. As I have said before there are FAR too many vents in this game that marines can get into (by themselves and have no support, i.e. jet packs and or buildings).
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The real problem is that it lets marines move very quickly between areas they are not meant to traverse so rapidly, and in relative safety. The route from Skylights -> Overlook and Topo -> C-12 on Veil illustrate this perfectly. Both vents are very easy to enter, and cut off a significant amount of travel time to get from one location to another. You can beat a spinting marine running the normal route going through both of these vents- and not only do you get there faster, you are going to do so silently, and you will be able to win most of your fights. Compare this to the marine sprinting down the hall, who is noisy, easily ambushed, and will not have the draw on any alien he encounters- there's no reason not to go vents.
This is especially bad for the aliens because the whole point of having vent systems in the first place is to give them a mobility advantage in the map to help counter things like phase gates and beacons. When the marines can use the vents as well as the aliens and win more of their fights within, the vent system ceases to become an advantage.