You're suppose to fall to last once you get around 85% (provided the other team doesn't fuck up ofcouse), the idea is you defend last with uber as opposed to no uber if you just stayed at last and let them get cap spire + move lobby. You're in a really bad position if their medic climbs spire and they attack thru top lobby, with or without uber.

I don't see any advantage of holding top lobby uber v uber compared to holding last point.

I guess the exception is you get some kills when they try to cap spire / move into lobby and want to retake spire -> see provided the other team doesn't fuck up.

Also take a look at bowts posts if you haven't already, guy's a genius.