The only downside I can think of is it seems to be easier to drop the chain on a triple. On my first bike with low-end Shimano Sora I experienced this with some regularity. My current bike is a triple with Shimano 105/Ultegra and I have only dropped the chain one time that I can remember in 4,500 miles. Keeping the bike tuned and the chain lubed has kept the triple working just fine for me.

Of course I didn't have the budget to get my bike any other way, the next choice was full Ultegra with only a compact double and a $400 price tag increase.