Well. First, making vampires with souls cheapens Angel. One of the things I liked that Blade The Series did was have Krista turn This made it clear, Blade's serum barely works, Krista is an high functioning depraved addict who will most likely be consumed by the thirst, and any attempts to save people won't work, even in a series with natural vampires. That type of thing may work in video games, but it takes the wind out of drama. It also destroys one of the arch-themes. Growing up is good. Change is good. Learning to let go after loss is good. A vampire has been deprived that. There's also a pretty good chance it may lead to the damnation of your immortal soul, as well as Wllow's, as it's not white magic. Magic comes with a cost.
Two, what stories can you tell with it? So, Tara is a lost soul on a fruitless path for redemption? Tara is an evil half-ninja ex-girlfriend out for revenge? No, she's just someone who was dead for a few minutes, before Willow hacked her. There's no drama in making her a ensouled vampire, unless you want a class change. And if you want that, do that right way, like Illyria.