I think it is the same misguided discussion as strongly, staticall typed vs weakly typed languages.
(Mind I mean the discussion is misguided, not you)
The problem is that relational model is a one solution to certain problem of when you want to pay the price of enforcing the model of your data. Relational database is when you pay all the cost upfront.
With MongoDB I have other options.
It does not mean they are necessarily better options. But some of them are better in some situations, for example when rapid prototyping. Or when you need to preserve data in different formats as the application evolves.
Personally, if you can, you probably should model your data upfront. But it is good to have other options, sometimes.