Definition of manifolds with finite atlas It is known that any manifold of finite dimension $n$ has an finite atlas (with at most $n+1$ charts). 
My question is then the following : 
why don't we define an atlas of a manifold (in the finite dimensional case) as a finite set of charts that covers the manifold,
for teaching purposes. 
Wouldn't that simplify the presentation and the theory (partition of unity, etc.) ?
Or in other words : do we encounter naturally manifolds with an infinite atlas (as part of their definition) ?
There's a relevant discussion on pp. 12-13 of Introduction to Smooth Manifolds by Lee:

"Our plan is to define a "smooth structure" on $M$ by giving a smooth atlas, and to define a function $f:M \to \mathbb R$ to be smooth if and only if $f \circ \phi^{-1}$ is smooth in the sense of ordinary calculus for each coordinate chart $(U,\phi)$ in the atlas.  There is one minor technical problem with this approach: In general, there will be many possible choices of atlas that give the "same" smooth structure, in that they all determine the same collection of smooth functions on $M$.  For example, consider the following pair of atlases on $\mathbb R^n$:
\begin{align}
\mathcal A_1 &amp;= \{ (\mathbb R^n, \text{Id}_{\mathbb R^n}) \} \\
\mathcal A_2 &amp;= \{(B_1(x),\text{Id}_{B_1(x)}) : x \in \mathbb R^n \}.
\end{align}
Although these are different smooth atlases, clearly a function $f:\mathbb R^n \to \mathbb R$ is smooth with respect to either atlas if and only if it is smooth in the sense of ordinary calculus.

"We could choose to define a smooth structure as an equivalence class of smooth atlases under an appropriate equivalence relation.  However, it is more straightforward to make the following definition: A smooth atlas $\mathcal A$ on $M$ is maximal if it is not contained in any strictly larger smooth atlas.  This just means that any chart that is smoothly compatible with every chart in $\mathcal A$ is already in $\mathcal A$.  (Such a smooth atlas is also said to be complete.)"
The oriented surface with infinite genus does not have a finite atlas.