However the user may wish to change - among the proposed search engines (ahem... imposed) - only the domain from google.com to google.es or .fr, or .it only to have a better territorial response in the query results.

Always using google, the risks to the user of incurring malware/adware hijacking, would remain the same, indeed, it would certainly be minor to that in which the user could incur, installing for eg. an extension not previously checked by google. As sometimes happens. . .

The gain of Opera for the use of the search engine would remain unchanged, I think, because the user will always use google, but the user would have a service more targeted to his needs.

I hope I was clear.

Greetings . . .

So, what is the purpose of the "Other search engines" feature if these cannot be used?

It allows you to set up other search engines (not on the original list) with keyword letters that can be used in the address box as prefixes to force their use in searching. Restricting default search engines to a preset list reduces the chances of malware/adware hijacking the browser default search function (to prevent searching for a hijacking remedy) and as a means of obtaining revenue from renting favored placement of the listed engines.

So, what is the purpose of the "Other search engines" feature if these cannot be used?