...why should he as a small supplier give discount?
To get the business?
A lot of group buys mean only saving a couple of quid so is it worth the hassle?
That depends on your point of view. Your point of view seems to be biased towards the trader. Speaking as a trader I'd have to say that I don't need people to express my views for me, I can do it for myself. A lot of businesses (mine included) operate in the face of some very stiff competition. For over 25 years in the office supplies business I've been competing with likes of Viking, Staples and lately even Tesco. Practically every deal that I do is negotiated. That's what business life is like I'm afraid and if it weren't, you'd probably be among the first to say that the market was rigged.
If you check my recent posts on the forum you'll see that I'm not unfamiliar with group buys and I know that they can give people hassle -- especially the poor suckers who manage them. That's partly down to the fact that only about half the people who join a group buy appear to read what the organizer writes, and partly because people are, well, prone to events. If at the beginning they just did as they were asked, and at the end just did what they'd agreed to do, everything could run very smoothly and the biggest issue then (apart from the limitations of the forum software, which I'll leave for another time) would probably just be the availability of supplies.
But I really don't see that there's a problem with group buys as far as the trader is concerned. He can get more turnover in one transaction for less work. I'd LOVE it if my customers would think about what they want each month, write it all down, and then send me a list of their requirements with product codes, quantities, delivery addresses and dates.
Instead, they call one of my staff and spend half an hour on the telephone explaining that they don't know what they want because they haven't even looked in the catalogue that I bought for them, then they ask for quotes for three completely unrelated alternatives, and then they go away and buy something completely different from a competitor.
If anyone would like to organize a group buy for office furniture ... oh, sorry Tony, scratch that.