That's all well and good, if the board lines up that way, but there may not be a defensive player worthy of the pick at #28. I know the response to that may be to trade back, which would also be an option, but you have to have a trade partner to do that AND you have to consider the grade on the player you are passing on at the alternate position to move back, but sure as hell don't reach.
By way of example, when we were on the clock looking for a defensive player the year we drafted Martin, the smart move was to take the guy who became a pro bowl/all pro type of player. You can't pass on that level of talent, or shouldn't unless you get serious compensation by trading out of the pick.
If you don't like the options on the board when you pick, then you try to trade back even if you don't really get full value by trading back. It should be all based on th grades you have on the players and there really aren't complete absolutes, obviously, but you want value for the draft picks even if they aren't ideally the position you would want to use the pick on.