| You asked about vents. On a two bath house, as a rule you have three vents. One each on a bathroom lavatory that vents the group, and a kitchen sink vent. If you have a laundry room that could be another vent. Sometimes we tie them all togather in the attic so that we only have one pipe comming out the roof. This is called reventing. It is possible that the bulder terminated the vents in the attic but I don't see how that would pass inspection. A vent that does not terminate through the roof is called a local vent. I am giving these terms so that when you talk to your plumber friend you will be on the same wave length. There is only one way to prevent sewer gas fron escaping from a local vent that terminates inside a wall. This is a mechanical vent that is spring loaded so that it lets air in to vent but stops sewer gas from getting out. This vent is called a Studor Vent. A plumbing drainage system has traps on each fixture. These traps keep sewer gas from escaping into your house. On a unvented system the water draining sets up a suction that syphons the water out of the trap letting sewer gas out into your house. I have tried to explain how vents work and the different kinds. You asked how important venting is. Vents are necessary for a plumbing system to function.
"When I use the washing machine or the shower a horrible sewage smell appears. It seems like it sucks the water out of the bathroom sink. A gurgling noise is heard and then the smell appears."
What's happening is the lavatory vent in the bathroom's blocked setting up a vacume that sucks the water seal out of the trap allowing sewer gas to enter your home. The solution would be to snake from the lavatory roof vent and clear the vent. Good luck, Tom | 
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