Two comments.
1. If lightning strikes the 100' tree next to your antenna, there will be enough current flowing into the nearby antenna to fry your equipment.
2. Just because you don't ground your antenna, lightning will easily find a path to ground anyway. Most often this is through your equipment and power, phone, and cable-tv connections. This will not only fry your equipment, but quite often burn your house down as well. Grounding the antenna is simply giving the lightning a fairly direct path to ground instead of through your valuable equipment. It will also tend to slowly bleed off current building up to help prevent a lightning strike (although don't ever count on this happening 100% of the time).