You cannot use a relate or relationship class to do symbology of a 1:M relationship. As far as symbolizing you have to convert the 1:M relationship to a 1:1 relationship. This can be done either by using summary tools, or if you have ArcGIS 10.1 or higher, by putting everything into the same geodatabase, joining the features on one field, and exporting the features to create a 1:1 feature class.
IF you are querying fields from only one data source in the relationship, then a relate works fine to select the records in the related feature class. However, the methods mentioned in the first paragraph are the only efficient way to query fields that involve the combination of the two datasets.
Of course, both of these methods do not refresh if you edit the original data, so they constantly have to be recreated.
If you are willing to use an Inner Join (only records that match in both data sources are visible) you can use the Make Query Table/. But that tool does not refresh if you edit the original data, cannot be edited, and performs badly, so it sucks. I always use the other methods I mentioned and never use this tool.
For some queries and analysis you can use cursors and dictionaries to perform the join in memory and get results, but it won't generally help with symbology without creating a new feature class..
