I never thought about it before, but I guess when you take into consideration that Buffy didn't know what the amulet does it wasn't a very promising plan. The slayer-team downstairs was outnumbered, and the gang upstairs weren't exactly the most fierce warriors to be taking on ubervamps by any means. But then again, maybe we're not giving Buffy enough credit for this one. Here, I'll explain...
Giles, Xander, Willow and Anya-- these are people that Buffy's spent the last 7 years fighting beside and who have obviously (over the years) learned to take care of themselves in a fight. They survived the battle at Sunnydale High, the Initiative, Glory and worse coming after them.... maybe Buffy wasn't so crazy for trusting that they could take on a few ubervamps each with some preparation.
And when you think about the couples that they had set up on the main floor, it would make sense to put a "vetran scoobie" at each exit. The scoobies weren't *all that* famillar with Wood or Andrew since they're new to the season (and Buffy was clearly concerned for Dawn's safety) so perhaps she wanted to have someone she trusts to look after the exits upstairs. Someone who knew the kind of hell Sunnydale could dish out first-hand to look after the new characters.
Now for the basement crew, Buffy did have the plan to activate a whole lot of slayers. And okay, it looks like they're still outnumbered. But let's remember some of the impossible odds that Buffy had taken on over the series... and won. She found a way to take out a gymnasium full of vamps at her old school, killed a gang of demons in a hell dimension at age 17 ("Anne", Season 3) and has taken on powerful vampires more than once (The Master, Angelus, Dracula). If that's the kind of odds that one young Slayer can face alone: a team of two trained slayers, a deadly vampire and a dozen new slayers may have stood more of a chance than we give them credit for. Especially with an uber-powerful witch upstairs, who presumably could have joined in the fight later on if needs had be'd.