It's a tricky question but will give it a go.
People go to northern soul nights to hear records they like , the more they like the better the night is for them so they are likely to attend again and tell others so increasing the numbers to keep the event going , it doesn't matter if they hear it off a boot , reissue etc
I have seen some top dj's from back in the day play from cds , Colin Curtis , Kev Roberts Brian Rea etc and the floor was full .
One dj I know has a massive collection of tunes , all the greats and has played at all the large events but he includes one record in his set usually that packs the floor but it is not an original , very few people know this and even less seem to care , he said he can not get an original as its very rare £3k but has the means to get one should it come up , I only found out as I went to ask if he would play the flipside for me and saw it was on a carver. It is the only non original I have ever seen him play in 15 or so years.
It seems that the ones who moan about it either want their shot at djing and can't get a booking or are people who want to keep djing themselves and dont want other people coming in stealing slots .
I also think that the over pricing of records does not help , what is worse a dj playing a £10 boot of a £100 record or a dj playing the original of the £100 that he has just had to pay £250+ to a dealer for.
Nobody needs to play bootlegs but nobody wants to see an empty dancefloor or have to pay a fortune for 2 and a half minutes of sound on a 7"single , is it just being lazy and not finding your own originals?
People will play what they want , sometimes try to deceive others with look a likes , promoters wont mind if numbers are high and dancefloor is full.
Obviously there are a few events that you wouldn't expect boots to be played at , those like the 100 club with good reputations but if its a reunion of an event from 40 years ago and people want the original djs , some may not still have the tracks played then as musical taste and situations change