Detective teams and Romantic involvement

This post is inspired by my current read, THE VARIOUS HAUNTS OF MEN by Susan Hill.
It is labelled as a Simon Serrailler crime novel - in fact it is the first in the series.
Detective Sergeant Freya Graffham has recently joined D. I. Simon Serrailler's team in the cathedral town of Lafferton and has realised that she has (almost at first sight) fallen in love with him. She has found out through a mutual friend that he has this effect on all women, and despite apparently extending friendship, never commits himself to romantic involvement. I still have about 200 pages to go, so am now wondering how Freya is going to resolve this. It is the last thing she needs at the start of her career.

The second novel that springs to mind is PHOENIX by John Connor. I am actually listening to it on CD and am about half way through. D.C. Karen Sharpe has newly come to the Pennines from London and joined a team being headed by D.C.S John Munro. Munro has taken up waiting outside Karen's house for her to come home late at night. He is obviously very attracted to her and she to him. They are sharing confidences but any relationship is in early days. Romantic involvement with him seems to be about the last thing she needs. She seems to have some deep dark secret in her past which is likely to affect any relationship she has.

Now the other example I have in my recent reading is CLEAN CUT by Lynda la Plante. This is the third in the Anna Travis series. Anna's romantic involvement with D.C.I James Langton began early on in ABOVE SUSPICION, the first in the series, continued in the second THE RED DAHLIA, and by CLEAN CUT they are living together, although they have not been working on the same cases for about 12 months.
When Langton is horrifically injured in a murder arrest that goes wrong, Anna becomes vital to his recovery. While Langton is still in hospital, Anna is assigned to a new case where a librarian is found dead by her twelve year old daughter returning from school. The suspect in this case is yet another violent rapist released from prison far too early. And then, perhaps a little predictably, the case Anna is working on and the one Langton was working on when he was attacked become linked. This a long and complex novel, with an ending that ensures there will be yet another sequel.

What other examples can you think of?
What is leading modern writers to include this sort of element in their plots?
Is it to make the point that romantic/sexual involvement between members of a team can impair sound decision-making? Surely we already knew that.
Or, and this may just be the cynic in me speaking, is it to make the crime fiction genre more palatable/attractive to those who like some romance/spice in their reading?
My other question relates to whether the author is assuming that readers will consume these books in order. Perhaps they are aiming at capturing an audience and then working within the framework that that audience enjoys.