The problem with the joint purchase agreement is enforcement. You could all sign the best, most airtight legal contract ever written before the equipment is purchased, but if at the other end someone does not go along with what the contract says, or "disputes" it, then enforcement comes down to a legal matter. If the dispute qualifies for small claims court, then it might be able to get legally enforced without too much cost, just some time. If it does not, then you are left with having to get lawyers involved and filing. Now you are looking at some bigger money up front for the legal support. Would only be worth it if you are talking about one heck of a lighting or PA system! IMO, agreements are usually only worth anything if all parties are in agreement at the end. Otherwise, it is still an argument. I am a big believer in each piece of equipment being owned outright by someone. If it is a new band starting out with nothing, then usually the equipment needed to be bought can be split up fairly evenly as far as dollar value between members. If it is a band of more experienced players, they probably have random pieces from past bands already that can be pooled together to reduce the amount needed. If no one has money to buy at the beginning, then rent (as someone pointed out earlier). Then as members start buying equipment, you will have to rent less and less until eventually everything is owned.
I earned my living in bands from the late 70's through the late 80's, and this is my opinion based on that experience. Each person comes in with the equipment they own (or purchase during the time in the band) and when they leave it is theirs to take with them. As someone mentioned, most bands do not last forever.
enforcement = small claims court. of course, that will cause more hard feelings than one may be willing to deal with.