PB,
At first sight it looks illogical to try and reduce the time interval between take-offs by putting routes closer together. However, as someone has already mentioned, the increased accuracy of PBN means that route divergence for a 1 minute time interval could be cut from 45 degrees to around 20 degrees. The trials are collecting data to try and establish if this can be safely achieved.
If these trial routes are implemented, it would be possible when taking off from 27L/27R to go from 3 departures in 4 minutes with a combination MID followed by CPT followed by SAM to potentially 3 departures in 2 minutes. That is where the payback is for Heathrow.
From the point of view of pilots and controllers these routes are probably deliverable with appropriate safeguards. The only question that has to be answered - is the political pain worth the gain?