It will result in calculation expressions (those that refer to such fields) that are much harder to read as you have mentally parse out a more complex notation that FileMaker automatically inserts when you use a field with such a name. And GetFieldName can fail to return the correct field name--breaking the use of it when passed as a script parameter to a script design to modify different fields based on the field name passed as a parameter.
But why should your client be dictating field names? Field names are internal names that need not be visible to the client at all. Except for Table Views which really aren't the best choice for use by general users anyway, you can use the field tool to add a field to your layout and then use the text tool to provide text to label the field with any text you care to use. You don't have to use the defined field name on the layout.