Yep, I agree, an alarm/immobiliser alone isn't going to help much and certainly won't save you much in long run. However, it's worth considering as an extra thing to help thwart a tea leaf. Anything that makes things a tiny bit more hassle for them, costs them more time and so on, all helps. It might be enough to see them disturbed or favour another bike instead.
My Fazer came with the Meta 357T-V2 alarm already fitted, it didn't swing the sale, but was a welcome addition. The internally battery died when it hit 10 years old, but I replaced the control box for £120 (half hour job). Otherwise it's been fine. Had no hassles with thieves (touch wood), but I do always use a big 16mm chain too. Also, the bikes getting on a bit now and is usually parked next to something more desirable with less security!