A few years ago I read about studies that used these to help people regain bone density in the spine, and it seems that results showed they worked. For instance, in:
Jessup, J.V., Horne, C., Vishen, R.K., & Wheeler,D. (2002). Effects of exercise on bone density, balance, and self-efficacy in older women. Biological Research for Nursing 4 (3), 171-180.
I read that study and my recollection is that they worked gradually UP to 15% of body weight as the maximum. (They also did other exercises.) Benefits were great but they were most profound in those with the most bone loss.
Well, I considered getting one of these gizmos, and then I decided to use a small backpack instead. My reasoning was that a) I already owned a nice little backpack so it required no extra purchase, b) the dynamics of carrying weight on your back must be sound as humans have been doing it for many thousands of years (in fact maybe that's why they had great bone density and we don't), and c) it provided training that would later come in handy if we were to do some trail-trekking.
At that time I was pretty fit, and we were walking 4 to 5 miles every evening. I found that adding just a few pounds SIGNIFICANTLY increased the workout, in fact I didn't carry the pack the whole distance (we walk in a loop so I could drop it off). I think that it's really important to start low and work up gradually. Better to do it at a lower weight than to cause strain or an injury, because if you get laid up and can't work out for a while, that's not helping your bones.
I used weights pulled from my leg-weights and wrapped them in a towel so they'd be well-padded and not bounce against my back. Make sure the pack fits you neatly so it isn't sliding around and causing you to get off-balance. I don't see why that's not just as good as an exercise vest, and you already have it in your closet!
Thanks for reflecting upon your experience and your contribution to the conversation. Sounds like you have had some positive training experience with adding more load to your activity.
At the same time, one of the problems with backpacks, that the weighted vest concept sidesteps, is our already problematic habit of hunching forward. This is necessary to offset the weight of a pack on the back. Forward rotation of the shoulder and rounding of the spine contributes to many postural problems.
By distributing weight more evenly, it invites us to keep an erect spine and work on opening the chest and keeping the shoulders in alignment.
This is also one of the positive elements of the upper body carrying devices that crosses over one shoulder and over the front of the body. I have an Ameribag that I use for travel for this very reason - it is far easier to counter the forward pitch that backpacks can create.
I suppose you could create a similar effect by having a pack in front as well in back to distribute the weight.
Just to point out that added weight is not the only factor to consider.