If the plaintiffs are working on contingency, do you calculate an imputed cost? If so, you'll basically put an end to any semblance of justice for victims of medical malpractice, defective products, industrial polluters, etc.
A lot of jurisdictions, e.g. England, have a 'loser pays' rule (as the default anyway, it's usually at the discretion of the court), and don't seem to be particularly less fair to malpractice victims etc. than the US.
One reason may be better legal aid. But also, a claimant who wants to avoid the risk of paying the defendant's fees if he loses can easily do so, by doing what anyone does when they want to avoid risk: taking out insurance (here, against losing the case).
So if you lose, the insurance company pays the costs order instead of you. If you win, you pay the insurance premium out of the costs award you get. It's called ATE (after-the-event) insurance. So you can basically use an insurance company to nullify the loser-pays rule should you choose to.
Cost orders are on a standard basis, so don't change depending on whether a lawyer is working on a conditional fee.
The U.S. does a lot of things through private litigation that other countries do through administrative regulation/enforcement. E.g. in European countries you have laws and agencies you can turn to if your employer treats you unfairly. In the U.S., even things like racial discrimination in the work force are left to private lawsuits. Basically, there's no free lunch.