The weight savings wouldn't be much either, once you get all of the cells together for the capacity you need, they still have a decent amount of peukert, charging challenges, and lifetime issues if used in an EV fashion. As far as weight goes, you are closer to the density of lead-acid than you are to lithium. I'm using Panasonic 6.5Ah NiMh from Honda hybrid packs myself, it is only worth doing if you are okay with the capacity of them not being in parallel. In my case, I have an application that only uses them at 2C max and is fairly easy to charge since they are in a series string and I'm using very low current when charging them but I'm also using them in a fashion without monitoring where I might kill parts of the string with overdischarge since a few cells have less capacity than the rest after some self-discharge since the cells I have are discards from pack rebuilds so I have to use them directly after charge and overcharge the rest of the pack so it is all fully charged. Not quite the same situation inside a normal hybrid pack but there are challenges to using these small NiMh cells, whether prismatic or cylindrical. Life is easier with the lithium cells. Don't put those prismatic cells in parallel.
