Basemap is necessary if you travel abroad of your country and if in your MAP folder are maps from different providers.
The basemap is like a support(template) where the maps are fixed like in a puzzle and which is helping the GPS SW to could route to destination, if your route starts from one country and end into an other.
Generally if you use the maps from only one provider, Basemap isn't necessary.
On the other hand, the basemap is not slowing your device.
My advice is to have the basemap in MAP folder.