Well, avast! has Script Blocker since version 4.0, while Web Shield was introduced much later (in v4.6).
Now, Web Shield detects most things Script Blocker would have (including obfuscated scripts)... and much more. However, yes, there are also (minor, I'd say) situations when Script Blocker may detect something more.
In particular:
1. If the file doesn't come from web, but rather from disk (i.e. if you load an infected web page from disk, which includes browser cache - even though in that case you must have visited the site previously anyway), then it cannot be detected by Web Shield, of course.
2. In very specific cases (and I am not aware of any at the moment), it's possible that the Script Blocker detects a malicous script after decryption (if WebShield doesn't detect the encrypted parent)
3. Script Blocker works even for encrypted connections (HTTPS), where Web Shield doesn't see the traffic.