Genuine spares are designed and manufactured in such a way that it can only fixed in a particular model. These parts are tested very seriously and don't create problems often. In case of local available spares, it create problems often. In most of the cases, local spares are designed badly and untested. There's nothing wrong in installing a local spares but make sure that you are aware of its durability.
Safety purposes
For saving money, you can go for local or third party spares. Imagine you are cruising in a highway and suddenly the brake cable cuts are getting jammed. This may end up in accidents. Or imagine any other part fails during its operation. You are purchasing the original spares after the testing process and certifying by the manufacturer. I'm not telling original parts will not fail. But the possibilities are very low.
Updated
When a OEM sells their part, they always get suggestions and reviews from their customers and from their suppliers. So when a customer or supplier files any complaint, they immediately get on the work for improving the quality of the product. So that the same problems and complaints can be avoided in the future.
Reliability and value for money
We know that the original spare always costs more than the third party spare. But in a original spare, good quality of materials are used and tested multiple times. This make sure that the part performs good. Where in local spares, low quality of materials are used for cost cutting purpose. Here the durability and reliability of the parts becomes question mark. The performance of the part is also not guaranteed.
Performance
When you install a local spare, you may end up in damaging the quality of the machine. There might be chance for poor handling and performance downgrade also. To run your machine with optimal performance with no problems, you should always use original spares and accessories.
Warranty
When you get any part from the OEM manufacturer, you are assured with warranty. Once you get a original spare, you are guaranteed with replacement or repair. OEMs take full responsibility of the products that they sell to their customers. Where in the case of local spares, you do not get any warranty or guarantee. If you get any problem in your spare, you may struggle to get it repair either from manufacturer or from mechanics.
Availability
OEMs sells their parts and accessories via their showrooms. You can head up there for buying your part. Or you can get it from authorized spares dealer. When you order for a local spare, it takes much time to show up or you may get a damaged piece.