Since you are in a multiple occupant building, it is more than just polite to ground your antenna. It protects the lives and property of your neighbor.
That being said I dont have my roof mounted antenna grounded. However I have a giant metal roof and a huge oak tree that stands over the house....whose root system is my lightning rod.
Your "system of grounding" has worked so far, but it sounds more like a problem waiting to happen. Currently, your antenna may be a more attractive target than the tree. Also, a strike on your tree will likely adversely affect your system.
It is commonly known that one shouldn't seek shelter under tall trees in a lightning storm (because of the likelihood that a tall tree will get struck). If the tree were an effective lightning rod for protection, then we would head for
the trees instead of away
from the trees. To think that your system is safe is a misconception.
Many people think that grounding will save them from a lightning strike. It won't save you from a direct strike. It may lessen the damage from a direct strike.
Grounding helps to avoid a direct strike. Grounding attempts to make your antenna "electrically invisible" to lightning by making it resemble the electrical potential of the earth or ground. Without grounding, your antenna sends up "leaders" just as your tree, and everything gaining charge during a storm. Which leader the lightning chooses to follow is unpredictable.
Grounding does much more than most people think. Most have discounted grounding as nothing more than satisfying required electrical code.
Grounding provides a drain for excess EMI and Rf energy that collects on your antenna. As wind blows across your antenna, electrons are shed producing an electrical charge (static charge) that will build up on your antenna causing problems in reception. Unless this excess energy (noise interference) is "drained", it will cause problems. The ground is a drain for unwanted EMI and Rf noise.
Grounding is especially important in amplified antennae. When amplifying an antenna, an electrical charge is sent to the antenna. This will make your antenna MORE likely to be struck if it is not grounded.
Grounding reduces noise and increases avoidance of electrical interferences.
Here is some more: