50/34: Totally viable race gearing, particularly if there are any significant climbs involved. I find it comical that the best climbers in the world (Contador being the prime example) have no problem embracing a compact crank for mountainous races but so many amateurs will tell you that you must be weak if you run a compact. Where the compact falls short: Flat/downhill/tailwind sprints, certain descents. As a sprinter, I moved to a 52/36 because I found I was getting spun out in fast sprints. I do a local group ride that ends in sprint that typically has a decent tailwind. When the wind is strong, we hit decent speeds there. Last week 43.6mph was the top speed. With the 52, I can hit that speed at 118rpm, while it takes me 123rpm to hit that same speed with the 50. For me, it makes a difference. YMMV.
52/36: This is what I use. I’m a pretty poor climber (FTP of 4.0 W/kg), so I need the 36. If you are doing a lot of climbing with a 39, it is very likely that (i) you are either a very strong climber (FTP of 4.5 W/kg or above), (ii) you just don’t climb very difficult terrain or (iii) you should be on a compact or mid-compact. Again, the best climbers in the WORLD are more than willing to ditch their standard cranks when the terrain gets steep, but YOU need a bigger gear than they do? With the 52, you really are only going to get “spun out” under some unlikely scenarios, like >45mph sprints or certain descents. But I find that more often then not, on fast descents, you’re better off tucking and getting aero than you are trying to pedal at 45mph, and if you do need a little kick of speed, a nice short high cadence effort can get your speed back up, at which point you are better off tucking and enjoying the glide.
The pros can pick and choose gearing depending on what that day’s race looks like. For most of us, we have to make one choice and live with it. I would venture to say that unless you are a decent sprinter or an outstanding climber, the 50/34 (and in particular, the 34 little ring) are going to save you a lot more time on climbs than the 50 is going to cost you in high speed sprints or descents. And the smaller 50 front chainring allows you to stay in the big ring and maintain momentum over rollers and milder grades than would a 52 or 53.
Another thing to think about: The difference between a 53 and a 50 is 6%. Go take a look at your numbers and see if you are within 6% of a pro’s power output. So are you really so strong enough that you “need” a 53/39? Even when the pros themselves often demonstrate by their gearing selection that 53/39 is too much gear for THEM?