Personally, I would feel terribly uncomfortable if they had booked dancers like this for entertainment. It is a bad idea for two differing reasons.
1. I attend hackdays to build things. Not for loud bands, giant jenga or lap dancing. The intention is hacking, surprisingly. Organisers should focus their energy and resources on good wifi, nice apis, etc that appeal to the hackday audience instead of any form of auxiliary entertainment.
2. There seems to be a slight gender imbalance at hackdays. I don't think lap dancers are going to help that.
Even if this is the local 'norm', Yahoo hackday is a global brand. When people see behaviour like this, it marks the whole tech community as either misogynistic or ladish.