I think it affects the site (and members) in a few ways.
Overall, for impulse buys (i.e. if people are enticed to spend money they normally wouldn't have), that's money they won't have to spend anywhere else, but for the planned purchases it likely means people will wait for discounts and not spend that money with anyone until one pops up. Ultimately, though, it tends to mean more spendage would be pooled twoards a select few sellers, which - while I recognise brings buyers to the site - is not something I view as a sustainable longer-term strategy, and may ultimately have a detrimental effect on everyone (that's one of the reasons I run promos and sales quite sparingly, if they're run too often it can come across as a manipulative gimmick - note, I'm not referring to permanent discount promos, like buy X get X discount or what have you, that's not really any different from the permanent P&H discounts I have in place).
If you think about it, since eBay are funding the discounts, it's pretty much like eBay giving these sellers kickbacks every month for staying on the site, but in a way that benefits their stats and takes traffic away from others, as well. (Just as a somewhat-related aside, but an example of what I'm talking about stat-wise, often the promos - especially the site-wide ones - don't include postage, which encourages people to favour free-post items because even if a free-post one is a bit more expensive, the discount can mean it still works out cheaper than one with post added separately. That then creates manipulated stats regarding things like X% buyers buy free post items).
That being said, I often can not be bothered waiting for a promo to save a few dollars, and while on some occasions I've bought from one of the sellers eBay were providing discounts for, on others I've chosen different sellers whose products didn't qualify (usually for reasons like convenience, better history / higher quality product and/or service, occasionally price). No idea if I'm in the minority, majority, or just average though. 😄
