Members get to buy tickets for matches before STHs can buy guest tickets. That is pretty much the only difference and they are proposing to take that away so the Bronze members then have an additional 22,000 people able to buy match tickets at the same time as them.
The guest tickets I've bought previously were for Albion fans that can't justify the cost of Bronze membership as they only go to a couple of games a year. Not everyone has the time or money to regularly go to matches so I'm happy to help them out by buying them a guest ticket when they can go...
Whilst I do regularly buy guest tickets I'm more than happy for the Bronze members to get their chance first and then take my chances with what is left. Definitely don't want to pay £20 extra for the luxury of doing so.