Back in the day (as us aging hobbyists begin all our discussions) it wasn't uncommon to see entire armies based with a little gravel and flocking. Today, though, the bases of armies have taken on entire characters of their own. An unfinished base can detract heavily from the look of an army.
Here is the flying Daemon Prince of Nurgle I've been working on for the last couple weeks. The base I sculpted using green stuff, painted up and added some Nurglings for some character. Still, the base appeared to be sorta bare despite all this. However, after adding some flock and chopping up some Hobby Lobby flower bits, the base has a character all its own.
Here you can see the effect applied to 3 groups of 7 Nurglings. As you can see, the bushes are alien enough to fit in with the landscape already on the base while adding much needed color to the bases. While I used to neglect this portion of the army for a long time myself, I've found the bases should be treated as miniatures themselves. If you can do this, you'll be able to tie your army together while at the same time adding another aspect of character to your units.