Proper ventilation is healthy for a home or attic area. Maintaining vents is just a part of homeownership. Overtime roof vents may rust, corrode or stop working properly. Once this happens, they no longer cool the attic areas well.
If the vents are not turning, rain will blow in through the vent. One of the more common issues with turbine vents is that if they do not turn freely, rainwater blows in through the slots therefore wetting the attic area. Insulation may become damaged and occasionally the water will seep down and stain the ceiling area. Therefore, lubrication and maintenance of the vents is prudent.
Plastic covers over vents
In the old days homeowners would put plastic or canvas bags over their turbine vents in the winter, thinking that it will help keep the attic warmer and thus making the house warmer. This in reality is poor practice, because the attic needs to be able to breath. Restricting the airflow may create a trapped moist air condition in the attic, which then causes wood deterioration and may cause damage to the insulation and even allow for mold or mildew.
It is much wiser to add more insulation in the attic, than cover the vents.
Who to consult? Roofers and qualified handymen.
General maintenance is often handled by home owners and handymen. Repairs or replacement is often done by roofers or qualified handymen.