Year 7 parents evening-child needs to be in attendance?

It is standard at my school. Generally it has been good for my academic-strugglers to hear the teachers praise how much they try.

(Why are you disappointed? The only good reason is that they're not trying.)

However I have always managed to have quiet words away from DD with SENCO/pastoral/whoever if needed.'

I was against taking my son with me initially after not doing at primary (so I could talk about them freely) but was pleasantly surprised in year 7 (now in year 9).
The teachers give him constructive criticism, praise (even in his worst subjects but didn't ignore any weak points) plus what to work on next. He seemed to take this from them much better than me saying the same thing and for me it has been useful to have him there. It was also reassuring to see that they did know him as an individual. I think it is one of few opportunities in our school to get the one to one feedback (it's a large school).
There were a number who didn't take the kids though so it's whatever you're most comfortable with.

I think it's a very good idea to have your DC there-they are old enough to start taking responsibility and should be included in the discussions.
And I think if the school is asking for them they really should go. It is a part of taking ownership and maturing.

Often the teacher will look at DC and ask how they think they are doing as an opener.

I think you get a much more realistic picture with them there-on both sides! it is only to help and clarify and I think my DC benefitted immensely from the engagement.

You can have a chat before if there are any issues DC would like raised or supported about.

Personally I don't like taking my DC to parents evenings. I think it's better to have the conversation without them there. My DC's secondary school seems to expect you to take them and most children do attend, but I've never taken mine (I have one in Y7 and one in Y9) and I haven't been 'told off' for it.