Zombie, of course you would want a fair weather ally right now versus a zealot if one wants to blow you up and the other doesn't like you pink scarf , but a fair weather ally can be swayed over to the other side as well over time. People you consider as friends could progressively start distancing themselves from you due to their change in taste, fear of your behaviors, or other "perceived" characteristics that make them less inclined to be your ally.
A zealot is someone we know wants us dead or gone, we know where their heart is and with the proper care, we can either avoid them or stop them from taking action.
A fair weather ally is someone that might seem like a friend, you may even count on their support for Tom Daley's wet T-Shirts sales, but when the chips are down and you may need them to help you keep your adopted kids, allow you to stay with your partner at a hospital/hospice, or fight to keep your job due to homophobia with your boss, can you trust them to support you? or will they swing over to the other side?
California's Prop 8 taught us that despite the support, fair weather allies are malleable. Popular support has to be irrevocable and final to make any long term changes.