Let me first turn this around - what made you notice that they were broken? Was it because of an issue, or did you just happen to be up there and go - wow this isn't one isn't moving and wow none of the others are either?
For 95% of the homes I have seen, especially with a tile roof (which essentialy blocks radiant heat transfer & has its own venting) there isn't a need for "assisted" ventilation.
Might the attic be a fraction cooler with them - sometimes.
Where are your ducts located? If they are in the attic, the ventilators might help some, but it would probably be better to make sure they are well sealed & insulated.
Will it save you the $1600+ to replace them - not even close especially when you only need the AC 45 days a year.
For more information:
Read "How effective are the different attic ventilation methods? I read mixed reviews, and many have a commercial bias." a Q&A answered by Danny Kelly.