No, "oneof" increases type safety. "oneof" describes mutually exclusive fields. For example:

  message Filter {
    oneof kind {
      Tags tags = 1;
      IPRange ipRange = 2;
    }
  }

With this in place, a client has to check the type of the filter in order to access the actual filter:

  switch t := f.Kind.(type) {
    case *Tags:
      // ...
    case *IPRange:
      // ...
  }

Without it, you can get into a situation where it's possible to create invalid combinations:

  message Filter {
    string type = 1;
    Tags tags = 2;
    IPRange ipRange = 3;
  }

Now you can accidentally end up with code like this:

  f = Filter{}
  f.Type = FilterType_Tags
  f.IPRange = IPRange{}

or:

  if f.Type == FilterType_Tags {
    useFilter(f.IPRange)
  }