I took a quick measurement of the biggest dent, and it seems to be 1mm, maybe a little bigger give or take. The other two noticable dents are definitely not bigger than 1mm.
I can't really advise. The question is whether the internal cells are damaged or not. Unlike Lithium cells, they're not likely to burst into flames, but I wouldn't know what the effects might be.
Have thick gloves handy in case the dents have damaged safety devices and the battery gets hot and you want to remove it. See Table 8.0.1 in http://www.duracell.com/OEM/Pdf/others/TECHBULL.pdf (502 KB download). Even if the battery seems ok it could be internally dented and short later. But you can see from the table examples where the cells get really hot, the case gets hot, but no fire or explosion. Keep in mind that the cells could leak corrosive liquid if the seals are mechanically damaged, leading to a mess much later. So check the external appearance of the case several times a year for signs of leaks particularly around the external contacts.