I have always thought that one of your back of the pen guys should be chosen for his ability to pinch hit and play in the field, because it makes it much easier to build a bench - even with 5 slots instead of 4, it's still tough to build a good bench. A pitcher who can help has a lot of value in the NL.
As far as pitching depth, you can't afford to carry a legitimate top 5 starter in the pen as a long relief "break glass" kind of guy. And while you can stash injury rehab guys in AAA, usually they have an out clause in their contracts that kicks in by July - and no guarantee they bounce back.
Phillies will have enough young SPs at LHV to provide depth, the key is a good pitching coach who can help them progress. So often, the difference between an AAAA starter and a 4th/5th guy is simply a tweak by a good pitching coach, whether smoothing their mechanics to add 1-2 MPH to their FB or helping them master a secondary pitch from below to above average. The new ML PC seems to love the changeup, which I think is the key pitch in this day and age, because it makes even an average FB harder to hit (by throwing the hitter's timing off).
The key to good coaching isn't to perform miracles, but to make numerous incremental improvements up and down the roster to maximize the production of the talent on hand.