It gives a false sense of security that doesn't exist.
Because the source page was served over http, the source page can be modified by an attacker, making the script be loaded under ssl makes you think its protected. But its not since an attacker could just modify the script tag to remove the https bit in transit then modify the script thats now being loaded over http.
Does it help much? No. Should you use it? Yes.
Who still has pages served over anything hit HTTPS?
Penalties for violations can be huge, as much as 20
million euros or 4% of the company's annual turnover.
More HTTPS == Better. If you can load this over HTTPS you should, no matter what circumstance. The browsers iconography will handle notifying people when the context is secure or not.