It used to be that you'd bring up a youngster and let him get his bearings in the majors by pitching in relief. Not always, not every prospect, but it was respectable tactic, dating back to the old-timey days when rookies were downright hazed. In more modern times, Johan Santana is a classic example, though it was debated hotly at the time whether they took too long to convert him back to starter status, and one could waste time arguing that their decisions cost him a HoF plaque. Adam Wainwright is another, more clear-cut example, who came up to stay in 2006 and pitched exclusively in relief for the Cards, then converted back to starting and never looked back.
By contrast, starter-prospect-turned-reliever Trevor May remarked early in his career that he understood his situation by embracing the bullpen, and that it was too hard for a pitcher to switch to starting once he got accustomed to relieving.
There's a disconnect here. The game evolves, but what exactly about it has changed so that putting a guy in the bullpen in the interest of getting to the majors quicker constitutes a death knell for his opportunity as a starter down the road?