French Immersion. Should you? In plain English, it opens doors for your child.
I of course took regular "core" French in HS. Because of its similiarity to Spanish, I took that too and got top honours in both OACs. What came as a surprise to me, since I majored in biology after HS, was that I ended up taking French electives in university. My French ability, despite exiting FI in gr 8, was strong enough for me to do that. Those electives were critical to me because as a science student, there's some major potential GPA-killer courses that all science undergrads must take. For instance, 2nd year Organic Chem, 3rd year Physiology, etc. I'll be the first to tell you that in terms of the work/effort/time required, humanities language courses demand far far less effort than elective biological science courses. Like 1% the effort/time. No joke. So while my science peers were choosing between all-consuming 3rd year Immunology electives, my workload was diluted with "easy" time-saving French electives, giving me ample time to focus all my efforts on the necessary "hard" science courses I had to take. I can honestly say that I was able to maintain high grades in my science classes BECAUSE of the extra time those French electives allowed me to have. It was like having only 2 or 3 courses to study for, while others had to study for 5.
In addition to school, I worked part-time at one of the major Canadian banks in their customer service call center. It was a national call center so it fielded calls from across the country. The minimum qualification was an ability to take the calls from Quebec that were obviously 95% in French. So I had "this" as my part-time job going through university. I literally had, next to me, a U of T School of Management MBA graduate, doing the same job. That person was happy to get a "bank" job out of graduation, and yet I was, as a mere high school graduate, employed at the same position as her. And this was exclusively because of my French ability. Had I stayed in the bank (as opposed to go onto graduate school), I am quite certain I could've climbed the ladder.
French also opened doors for me to "travel" throughout my late-teens and early twenties. There's something called the Summer Language Bursary Program (I believe it's now prefixed with the word "Explore"). It's basically the government giving you a free trip to study a for-university-credit French course, at a Quebec/Francophone university in the summer. The bursary covers tution, food, and on-campus housing. Basically, you just have to get yourself there, but after that, all expenses are covered. No jokes. The only criteria for participation is that you are enrolled as a full-time high school or university student. Most people only attend once. I attended four times, once in Trois-Rivieres, once in Nova Scotia (University Sainte-Anne), and two times in Jonquiere. Given how many years I was a post-secondary student (8 altogether), finding 6 weeks out of 8 separate summers for a free fun trip to Quebec was very easy to do. And definitely a highlight of my life during that time.
I'm now a doctoral-level health professional. I routinely encounter patients with whom I speak French to (as well as Spanish, and a touch of Italian and Portugese). I have kept in touch with my FI classmates from elementary school, and those who kept up with it are either teachers or have nice government jobs. One person I met at one of the bursary programs was fluent in Cantonese and Mandarin, and was a journalism student. Given her basic ability in French, she's now an on-air cross-Canada journalist for a Chinese news station. Her "hook" was her very basic French ability. Others I've known, because of their French interest, have travelled the world as exchange students, flight attendants, etc. Of course, you can be an exchange student (or do the bursary program) without being in FI, but I think the chances are much less that you would.
Bottom line is - adding French, on top of your qualifications, will open doors and the world to you. If it's an option for your child, do it.
(As an aside, for those considering gifted vs FI, I'd still recommend French because of the "emotional quotient" issue of being in a gifted program. I attended a gifted HS. I felt gifted students, while smart, weren't ready to venture out of their coddled bubble and into the world, where people didn't naturally treat them as special. I think a lot of students who enter gifted education from an early age (e.g. gr 4) have a hard time later, in facing the reality that the world isn't designed to honor and cater to them. This "let down" with the world persists far past HS graduation. I'm pretty sure some of them never fully recover/accept it. I don't find the same with my FI classmates, since a lot of effort is needed to stay in the program, so no one felt/feels entitled at all).