I don't think they are required, and here's why (of course).
I run very long tracks, 60 feet is a short track in my world,
and I have straights longer than most peoples tracks. I reach Terminal velocity, apx5ft./sec before I reach the end of that straight, and Must Slow Down!
Unless you race on glass you will bounce and move around, even on the straights, so good luck staying on track with more speed. I found that people don't enjoy a track that is too hard, or too fast as to be undrivable, and that's no fun.
With that said I plan on buying an adjustable Slot Car chassis, and building something neat and fast, but it will never race the other cars. On that note, I bought the Truck a long time ago and it is too heavy to race against other cars, not because it's slow, it's not.
It's because it tends to knock other cars right off the track, very funny but not fun for others.
Last, if you mod a car, make sure and keep the Center of Gravity as low as possible, otherwise your car will wander all over, can't imagine an oversized motor with that configuration.