How can you have a project without models and models without schema? And so you have to change a schema; is there usually much difficulty? More than supporting multiple schema across a single set?
Even if DBs were still bad at migrating a schema (eg. table locking horrors), the problem usually only presents with millions of rows, which one presumably doesn't have in the early stage of a project -- "start[ing] a project." Nonetheless, even with millions of rows, there have been solutions for all the major RDB since before MongoDB.
data mining : you dont know how exactly how your data will look like , and how much datas you'll have to handle. And you are opened to new types of datas.
data logging : you dont need schema databases for that.
file upload : GridFS , ...
User generated content : save semi structured documents ( like a configuration , a spreadsheet , ... ) that need to be parsed but dont need a "schema".
True, typically you don't know in advance of a data mining operation. Nonetheless, you have some invariants that you know you'll need and these can be modelled with a schema.
Additionally, when attempting to put a model into production, you'll normally know what it is you need, and this can be put into a schema for ease of interoperability with other requirements (CMS, booking etc).
Even if DBs were still bad at migrating a schema (eg. table locking horrors), the problem usually only presents with millions of rows, which one presumably doesn't have in the early stage of a project -- "start[ing] a project." Nonetheless, even with millions of rows, there have been solutions for all the major RDB since before MongoDB.