On the Birthday issue, coz me and my mates are usually low on funds, it makes sense that everyone buys the guy who's birthday it is a drink. It costs say 6 people only £2.50 each, but for one person to buy drinks for 6 people is £15, which is a fair chunk. Then the birthday money can be used solely by the birthday boy on himself to get him absolutely smashed, which is the way it should be on a birthday!
I don't really have any mates who are mingebags i don't think, all the 20 and 30p's work themselves out over the months, and any mingebag tendencies usually have valid reasons behind them. Plus one of our mates is rich and loves the fact, so insists on buying bottles of champagne for all of us on nights out after exam results or whatever, and is positively insulted if you try to slip him a fiver!
Only problem I've ever had to do with tightness is that one of my mates works in a shop that his dad owns. A couple of times when we've been in there and tried to buy a sandwich or whatever he usually lets us off paying, but this girl he knows went in there and took a tenners worth of stuff up to the counter and was positively shocked when he charged her, she didn't even have the money on her. This wasn't stuff she needed urgently, like lunch or a drink or something, it was like magazines and other stuff. My mate couldn't believe that she was trying to take advantage like that, and qualifies as severe mingebaggery.