The trail maps (from what I understand) are USER submitted, STAFF verified GPS data sets. So, you are correct in the fact that there might be trails within a park that will not be covered in the GPS. Most likely the GPS dataset (gpx file) will not include ALL of the trails within a park because they are created by a user riding the route. However, often it is the most popular trail submitted – so if you are unfamiliar to the area it might be a good place to start
As for upgrading – if you are not comfortable reading maps (or there are none) and you want to make sure you do not get lost 5-10-15 miles from your vehicle, it could be worth it.