Private equity firms are so focused on the bottom line that they often engage in utterly destructive behavior. They take functional firms providing a living for taxpaying citizens and load them up with unnecessary debt. They decimate and offshore the available jobs, ravage the pensions and force workers to become wards of the state. Their asset-shifting is not much different than simple looting. And the weakened firm left behind often doesn't survive long. Their actions are little better than those of your typical arsonist, who, after all, is also focused on the bottom line.
As CEPR notes:
The question is whether the high profits earned by the partners are primarily due to increasing economic efficiency or to rents earned by dumping costs on others.
As noted here, it is standard practice for private equity to load firms with debt. This means that taxable profits are turned into tax-deductible interest payments. The difference can be a gain to Bain and other private equity firms, but it is coming at the expense of taxpayers.
In the same vein, private equity companies often in engage in complex asset shifting. This can leave a heavily indebted firm with few valuable assets. If it eventually goes bankrupt, the creditors collect little money because the private equity company has transferred the assets with value into an independent company. This can also mean big profits for Bain and other private equity companies, but this is not a gain to the economy.
Another frequent game of private equity companies is to dump pension obligations on the Pension Benefit Guarantee Corporation. The reduction in liabilities can mean big profits for Bain and other private equity companies, but does not provide any benefit to the economy.
These are the sorts of issues that appear in serious discussions of the benefits of private equity.