The dolphins took Mike Pouncey around 15 a couple of years ago, and at the time I wasn't a huge fan of it. Not only that, but they decided to put him at center which made me question it even more. Pouncey is now a top 3 center, and I absolutely love the pick. To me, if the player you pick goes on to be one of the best at his position, then he is absolutely worth that pick. Now if a Calvin Johnson type goes after him, then obviously you're going to be dissapointed. Also, when you're rebuilding, sometimes a safe pick is the better way to go, and guards are considered safer.
Also, I hate when people say that you can find guards later in the draft. Obviously thats true, but it is true for any position. I would be curious to see the probowl players of the last 10 years, and what round they were drafted in. I bet it is pretty similar for all players besides maybe quarterbacks.