In the overall scheme of things Buffy's resurrection was incredibly damaging. It led to not only The First getting an opportunity to overrun the earth with its evil apocalyptic schemes- slaughtering hundreds upon hundreds in the process- but then we got Twilight, the end of magic, zompires plagues the re-writing of magic, new more powerful vamps.
Though I pretty much agree as to the ultimate consequences of bringing Buffy back, in defense of the Scoobies, they had no idea the can of worms that they were about to open. All that they were trying to do was to rescue their friend. They weren't looking beyond that point. Yes, they should have, but they didn't. They obviously knew that, if Giles had found out about their plans beforehand, he would have stopped them, and by that simple fact, they should have known that what they were attempting was wrong and probably extremely dangerous, but they chose to ignore the fact.
And all because the Scoobies had to find a way to deal with, in all essence, a lame biker gang.
On the whole I was never really convinced that super-witch Willow, Spike and The Buffybot were somehow incapable of filling in for the slayer.
And I remain unconvinced. Being what we're shown Willow's power level to be in Season 5/6. She had tons of abilities she never seemed to use here- telekinetic use of a stake for example.
Add to that Spike (who had, in the past, being a match for Buffy or at the very least a newly sired vamp) and the Bot and bringing Buffy back wasn't worth it- at least not in the long run.
I got the feeling that the Buffy-bot was little more than a stop gap measure, more to cover up Buffy's death, in order to prevent Dawn from having to leave, than to patrol the hellmouth. It was a machine, plain and simple, and what's more, a machine used for very stressful activities that it probably never was supposed to be used for. Granted, it had what Warren and Spike considered to be Buffy's strength, and what they thought was her skill level in fighting, but she was primarily a sex toy, not an assault weapon. She couldn't have lasted very much longer, the way that they were using her.
As for Spike, as highly as I regard his fighting skills, he was, at that point, still a soulless monster who probably only stayed in Sunnydale because of his promise to Buffy to take care of Dawn. Patrolling with the Scoobies was just something to occupy his time and keep feed his craving for combat He didn't like the Scoobies, and they didn't trust him If anything ever happened to Dawn, he would have no reason whatsoever to hang around a place that almost certainly contained nothing but painful memories for him.
That leaves a super-witch, who, no matter how powerful she might be, simply couldn't have done the job alone, even with the questionable help of the other Scoobies.
No. If they needed a Slayer there was one chilling in prison they could've broken out if their need was so dire. A prison break seems like a lot less effort than invoking dark forces and returning the dead from the grave...
They did for selfish reasons.
I'm not sure that selfish would be the right word Ignorance, certainly, combined with genuine concern that their friend was suffering in some hell dimension, which makes their actions understandable, if not particularly prudent.
Also, at that point, I don't think that the Scoobies were all that convinced of Faith's rehabilitation, if they knew about it at all. Just as likely, they thought that she had confessed in order to escape the Watchers' wet team. It's quite likely, in fact, that the only thing keeping Faith alive was that the Watchers would have had too hard a job killing her while she was surrounded by prison guards. While protecting Faith wasn't very likely high on Willow's list of priorities, she, and Giles, almost certainly knew that breaking her out of prison would be pretty much signing her death warrant, which would, ultimately, leave them back in the same situation that they were already in: a bunch of slightly skilled (relatively) amateurs, trying to do the Slayer's job.