Is it bad practice NOT to delete unused database entities?
In case of tables, if it's not being updated you are safe to remove then and keep a backup (just in case).

Other objects like SP, Views and Functions can be easily removed.

Of course you can end up messing with relational integrity if you are not cautions in removing data. I recommend full regression tests over the system.

The advantages of removing unused objects is keeping the schema (and the system) clean and simple, this helps maintenance.

The disvantage is the risk of lost of essential data, damaging relational integrity and jeopardizing the system.

Also be aware if you remove a substancial mass of data it smells like trouble.

Edit

This is long comment full of "IMHO".
I'm a developer happened to get some dba tasks by accident. When they say "it's better let things as they are" it means "ok, cleaning the schema is fine but developers and testers will get they hands full of (unnecessary?) work and they are already full"

And that's true, in special if there acumulating garbage from years of "better let things as they are". Developers can need to remove references to that superflous objects. Testers must testify nothing ill blow up, etc.

At the end of the day someone needs to make the hard decision: To clean or not to clean, that's the question. If you can (do you have good documentation? have the time? Can alloc coworkers?) make a "possible impacts" report to your manager where you can pontuate the advantages, disvantages, risks and answer one little question: Are us just postponing a job needs to be done some point in the future?
Btw the short answer for your quastion is Yes!