Question

The purpose of an abstract class is not to let the developers create an object of the base class and then upcast it, AFAIK.

Now, even if the upcasting is not required, and I still use it, does it prove to be “disadvantageous” in some way?

More clarification:
From The Thinking in C++:

Often in a design, you want the base class to present only an
interface for its derived classes. That is, you don’t want anyone to
actually create an object of the base class, only to upcast to it so that
its interface can be used. This is accomplished by making that class
abstract,

By upcasting, I meant: baseClass *obj = new derived ();

Answer 1

Upcasting can be disadvantageous for non polymorphic classes. For example:

class Fruit { ... };  // doesn't contain any virtual method
class Apple : public Fruit { ... };
class Blackberry : public Fruit { ... };

upcast it somewhere,

Fruit *p = new Apple;  // oops, information gone

Now, you will never know (without any manual mechanism) that if *p is an instance of an Apple or a Blackberry.

[Note that dynamic_cast<> is not allowed for non-polymorphic classes.]