What is the effect of the BRDF denominator? I was reading a paper on BRDF. I've come across this formula :
$$ f(\omega_i, \omega_o ) = \frac{FDG}{4(N.V)(N.L)}$$ 

The (N.L) term can be cancelled by the cosine term which appears in the rendering equation :

$$ L_o = \int f(\omega_i, \omega_o) L_i cos(\theta) d\omega_i $$

What about the (N.V) term ? What happens if (N.V) = 0 ?
Firstly, I highly suggest reading Eric Heitz's paper "Understanding the Masking-Shadowing Function in Microfacet-Based BRDFs", which covers the full derivation of microfacet-based BRDFs. 

The $\frac{1}{4(N \cdot V)(N \cdot L)}$ term is a side effect of the derivation of the BRDF for specular microfacets. Specifically, it comes from the Jacobian of the reflection transformation. See the paper and/or Walter et. al's 2007 paper for more details.



As for your concern for a divide by zero, the definition of the rendering equation prevents it. Let me explain:

For $(N \cdot V)$ to equal zero, they must be orthogonal. In this case, the visibility term of the rendering equation will cull the cases at / below the horizon.