The upside of Fiddler/HTTP's use of strings rather than strongly typed values is flexibility, backward/forward compatibility, and simplicity of implementation.
The downside is that it makes writing FiddlerScript a lot more error prone than it could be.
By enhancing the script editor with autocomplete for strings (Flag/Preference names, HTTP header names) we can significantly improve developer experience.
An alternative to this (which would work nicely) would be to create a new class named MAGICS that has a number of members e.g. Prefs, Flags, and Headers and each of those members has these values as string constants. So then you could do e.g.
oSession[MAGICS.Flags.CertificateSubjectOverride] = "host.com";
This would mean that you get both autocomplete AND the ability to choose to break scripts when a given flag is no longer supported (by removing the constant from the MAGICS).