What are some commonly asserted employer rationales in requiring "speak-English-only" rules?

(This section taken from the ACLU Report on Language Rights and Language Discrimination.)

1. Employee morale: Employees may complain about co-workers speaking in a language they do not understand. However, this usually reflects underlying, more deeply rooted interpersonal tensions that are best dealt with directly, rather than by imposing sanctions that fall only on certain ethnic groups--and which would, if anything, increase workplace tensions. And even assuming that employees are in fact using their proficiency in another language to denigrate co-workers--i.e., that the problem is not imagined--there probably are more individualized solutions, tailored to particular situations, that are preferable to imposing a plant-wide rule that stigmatizes all who speak that language.
2. Efficiency: The argument that English should be enforced for efficiency reasons ignores the fact that workers are generally able to communicate more accurately and quickly in their primary language.
3. Safety: Again, in an emergency or other safety-sensitive situation, employees will generally be able to communicate more efficiently and quickly with each other in their primary language. Indeed, federal OSHA regulations encourage employers to administer safety trainings in employees' primary language, be that English or otherwise, to ensure maximum comprehension.
4. Improving employees' English proficiency: Although it may be a laudable goal for employers to assist their employees in learning English, requiring them to speak it or suffer discipline or termination is an extremely punitive (not to mention ineffective) means of doing so. Employers interested in increasing employees' English proficiency have profitably done so through other means, such as providing access to ESL (English as a Second Language) classes. Moreover, if the employees were hired for their jobs at their existing English skill levels, it would seem difficult to justify increasing their proficiency levels on business necessity grounds.
5. Distraction: Some employers claim that it is distracting for workers to overhear conversations in non-English languages they do not understand. This argument lacks a foundation in common sense, i.e., one is more likely to be distracted hearing a conversation one does understand, than a language one does not. If an employer was truly concerned about distracting conversation, the proper approach would be a rule against all unnecessary speech, not just speech in languages other than English.
6. Monitoring employees' work: This rationale assumes that overhearing what employees are saying to each other is a primary method of evaluating their work. But employers commonly use other methods to do so -- visual inspection, quality control checks, and so forth. In any event, it is hard to see why a speak-English-only policy would be needed for this purpose, since (if the employees speak English to begin with) it is unlikely they would refuse to speak to a supervisor in English about work-related issues.