As a mixed perennial gardener, I have complained about evident vole/meadow mouse consumption of some of our spring flowering bulbs; notably, crocuses and daffodils.
At the same time, I've also complained about them not eating the Chionodoxa, Puschkinia and Scilla which aim to seed all over the garden.

More seriously, the effect of voles on soil fertility is apparently just now beginning to be studied. A recent Oregon study on the agricultural effect of gray-tailed voles found that voles may enhance soil nitrification.

Personally, I've also felt that with our (albeit upgraded) clay, the vole activity may aid in the aeration of the soil and in the permeation of rain and sprinkler water through it.