Let me take a crack at this, starting first with your general question:
If an adjective is removed, what's the consequence?
In some cases, there would be little loss of meaning, but the writing would be a lot more colorless; for example:
The hungry girl devoured the piping hot food.
The girl devoured the food.
Other times, the lack of an adjective will introduce ambiguity:
The tall policeman put a ticket on the red car across the street.
The policeman put a ticket on the car across the street.
which might be a bit confusing, particularly if there were two policemen (one tall, and one short), and two cars (one red, and one silver).
Now, to your question in particular:
The Congress shall have Power To lay and collect Taxes, Duties,
Imposts and Excises, to pay the Debts and provide for the common
Defence and general Welfare of the United States;
To borrow Money on the credit of the United States;
To regulate Commerce with foreign Nations, and among the several
States, and with the Indian Tribes;
To establish an uniform Rule of Naturalization, and uniform Laws on
the subject of Bankruptcies throughout the United States;
To coin Money, regulate the Value thereof, and of foreign Coin, and
fix the Standard of Weights and Measures;
To provide for the Punishment of counterfeiting the Securities and
current Coin of the United States;
Notice how often the term the United States appears in Section 8. That's because, for most of that section, the document is addressing federal issues: national defense, national debt, punishments for counterfeiting, etc. Yet in the commerce clause (the one you quoted), the Constitution is referring to commerce between the individual states, not just between the country and other nations. Hence, the United States becomes the several states.
Could the word "several" be removed and the original meaning stay preserved? I believe so, particularly if the preposition "between" was used instead of "among." After all, the document is merely enumerating three possible areas of commerce regulation, and declaring that Congress has the authority to regulate all three:
- Commerce between the United States and some other nation (e.g. The
U.S. and France)
- Commerce among the states themselves (e.g.,
between New York and Rhode Island)
- Commerce between the U.S., and
one of the Indian Tribes
Could that still be inferred and understood, after the word "several" was removed? Probably.
Bottom line: I think the word "several" helps convey the full meaning and intent of the clause, but I don't feel it's a necessary word.