The power and usefulness of this powerful tool are mainly due to its versatility.

Couldn't "is" be used in place of "are"? Are "power and usefulness" functioning as a plural subject? I'm just confused. Please guide me. Thanks.
The subject is plural when two nouns are connected with "and"

Jack and Jill (they) ran up the hill.

Here is a good resource for you to review subject-verb agreement:
http://owl.english.purdue.edu/owl/resource/599/1 /

Note also that the repetition of power is poor style in your sentence.

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Thank you, Alphecca, Clive.

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