I've never understood this concept.
I went out for a cider with some friends last night (OK, maybe more than one cider), and we were sitting next to a table that had a couple of it. After about an hour, my mate pops up and says to this table "Guy's, if you wanna talk about DJ's, he's your man!"
Cue my "Huh?" face!
Turns out this couple were talking about their wedding and were discussing the evening entertainment, which is what my mate overheard. After they enquired why they would talk to me about such things and my mate explaining (with me thinking "Shut up, it's my day off!"), they come over and asked me for some advice.
So here's the scenario: they've booked a DJ. Contract signed, and they are happy with what they call 'the basic package'. Now that the contract was signed, the DJ is upselling and this is what they are unsure about. They asked about uplighting, dancefloors, smoke machines and lasers. I explained what each meant, got some pics on Google and showed them how it could work out. Also explained that smoke machines may not be allowed by the venue, with means there is no point in the laser either. Never once gave my opinion - I just gave them the info and left it at that (after all, it's not my job to see it to them, is it?).
Then came the optional extra of a second DJ. "Should we consider it?"
"Well, that depends. Is the second DJ Calvin Harris?"
"No."
"Steve Aoki?"
"No."
"Roger Sanchez?"
"No."
"Tiesto?"
"Ok, we get your point!"
This was the only part I gave my opinion on, and it is very simple: either the DJ is not good enough to pull off a successful gig on his own, or he is but then paying for a second DJ would be a waste of money because you can get a great night with one DJ.
Am I alone in this thought process? If you offer the 'second DJ service', maybe you can explain what I'm missing?