Bark is a very good material on which to grow mold. Gravel mulches are good because they don't encourage mold growth. No white gravel. Flat stones and pavers work well for this too and in the right spot, they look good as well. Mulch holds down weeds and cuts down on summer water loss.
Earthworms often thrive under mulch and in general mulches usually help plants grow better.
The one spot where mulches are less effective is in those cold, always shaded areas. Here mulch will keep the soil from ever warming up. Every where else though, mulch is useful. Buggy Plants and Mold Plants that are not being grown right will usually get infested with insects. The molds then start producing spores and pretty soon there is a serious allergy situation in the landscape.