Pros: balance, confidence, good use of energy
Cons: have them wear cheap shoes
Striders and Early Riders (wooden) are the two most durable, well-designed run bikes on the market. We actually have the Early Rider. We got it for my son for his second birthday. It has a short enough stand over height for most 2 year olds. Something like an 11"" inseam if I can remember right. My son (4) now rides a peddle bike, without training wheels, but still rides his run bike on the grass and other places a peddle bike doesn't go well. This is the way to introduce a child to bicycles!! The Strider is a good design, real wheels, low stand-over for the youngest of riders. If ours had a bike computer on it, I think it would read that he's ridden it over 200 miles in the past 2 years. It's a great source for him to expel energy, and exert control appropriately.
The only con this approach to riding may have is that their shoes are their brakes. We live in a hilly area, and he didn't go through that many shoes, but the shoes he did have could not be passed down to another child, the soles were well worn. Small price for the best toy in the garage.