Some cases, implies most cannot as some often refers to a smaller number, it wasn't the word "most" which would mean lots do but some, implying not many at all. A manifested spirit can wield force powers then, how many force ghosts are there and how many have any useable force powers that would effect how things play out. Like i previously stated, the immortality is essentially a non-issue as the forerunners are capable fo rendereing them obsolete, whether by placing them in a temporal rift causing time to slow down for them so that they'd essentially be useless, or placed in slipspace and rendered inoperable to the rest of the world. My previous point still stands.