What is a correct argument for using a two tailed test even if there is a clear basis for predicting a result in a given direction?
If the null hypothesis is in fact true, a failure to reject it will give stronger evidence in support of it.
One tailed tests cannot be used in almost any real study involving two groups.
If an unexpected result comes out opposite to what is predicted, it does not have to be ignored.
A two tailed test gives you a better chance of getting a significant result.