I think it's as simple as replacing a lightly used RHB with another one. Rojas is actually more useful than Harrison given they already had two RH 3B and Harrison wasn't very good in the OF. He'll play less than Harrison but Thomson loves to play as many RHBs as possible against LHP so he really could start two of the next three games (for Marsh not Pache).
It may also tell us that Cave has not ever really been a consideration to return to LF regularly, partly because they already know what he can('t) do in MLB and and partly because they need a RHB (especially if they want Hall to stay on the bench).
Either way he's probably only up for between 6 and 16 days. Unless he thrives (and isn't traded).