Some short thoughts on ratings:
1) Overnights do matter. There is little doubt that being pushed against BGT and some decent weather after a lengthy winter is going to deplete the overnights, and not all will be made up on timeshift (casual, 'because it was on', viewers don't tend to timeshift).
2) Overnights don't matter more than timeshift and VoD viewers. They all watch, they all count.
3) Overnights do not tell you about 'appointment to view' figures. They are a mix of live and early timeshift viewers. A substantial percentage of the overnights are not live viewers sitting down to watch appointment TV.
4) We really shouldn't be concerned about the shift from overnight viewing to later viewing, when the consolidated ratings have shown it's not harmed the viewership levels.
5) We should recognise this though - the majority of all the casual viewers come from live overnights. The majority of timeshifters are people with an express interest in seeing the show. Research tells us that timeshifters tend to be keener, watch more, and stay more loyal, than live viewers. They also give higher appreciation scores. That seems more valuable than 'appointment to view', in an age where that is starting to matter less even for talent shows.