Database constraints - keep or ignore?
If somebody told me to ignore keys and constraints on my databases, I would promptly ignore them and go about my business.
Primary keys, foreign keys, and constraints are there for a reason. Use them. They'll make your life easier and your database easier to understand (and, quite often, more performant).
The longer I work with databases, the more I appreciate constraints. In the long run, they save me a lot of time. Only trusted constraints ensure 100% validity of data.
I wrote a chapter on usage of constraints vs. other ways of ensuring data integrity, available as free download here
If you think that constraints are merely about primary keys and foreign keys, then in fact you haven't been taught much of the "fundamentals" to begin with.
I suggest you take a look at "An Introduction to Relational Database Theory" by Hugh Darwen, which is available freely online. And at least you get a genuine education about the "fundamentals" from that one.