If a team has a starter/reliever type, it stands to logic that that pitcher has the sort of stamina to go 5-6 innings. The BFA number is based on typical usage. For example, Tim Wakefield has the capability of throwing 120 pitches every time out, but since he was used in relief a majority of the year, PB takes that into account and limits his everyday BFA's (22) to reflect his MLB usage. If a team starts a relief pitcher, they expect him to go more than his usual 1, 2, or 3 innings, but then he also becomes unavailable for the next 3-4 days...
Adding BFA's is the only way to reflect the capabilities of an occasional starter, while maintaining his "typical" relief usage limits...