Shakespeare's "Sonnet 18" is suitable for an oral recital because the underlying metaphor comparing the eternal summer of beauty to the eternal summer of beauty in a written sonnet is at once familiar and yet unusual; therefore while it can be followed in an oral recitation because of its familiarity, it is also thought provoking for its unusual aspect. Another reason it is suitable is that there are not interconnected strings of word play as in "Sonnet 35" in which a single word play string comprises four lines; thus this sonnet will be easier than some to follow and attend to, as well as being easier to adequately recite. Another reason is that the turns at lines 5 and 9, turns (or voltas) that are definitive of sonnet structure and descend from Petrarchan sonnets, are clear as they lead into a change in subject within the sonnet's topic; thus in an oral recitation, the turns can be dramatized and emphasized. A final reason is that "Sonnet 18" is one of Shakespeare's more elegantly beautiful sonnet's as he isn't lamenting or scolding or trying to forgive someone stubbornly not interested in forgiveness.
1) Because you like it.
2) It represents a good challenge when reciting its iambic pentameter structure.
3) Shakespeare's sonnets are some of the most influential poems in the history of poetry.
4) Sonnet 18 discusses the eternal theme of aging and the relentless passage of time. It considers the idea that the only permanent beauty is art (and therefore the only true beauty?)
5) It will lend gravitas and cultural worth to the recital and reassure the listeners in the sophistication of the occassion.
6) It is one of the best loved poems of all time.
7) It may awaken a member of the audience to the wonder of Shakspeare's work and so encourage another person to discover his priceless work.
8) The 'value' of a poem is not empirically quantifiable. A poem is not a government form. It is not be necessary to 'justify' the worth of art to the authorities (ie. your proctor). This hierarchical approving of art leads to artistic stagnation and censorship. Ultimately, you wish to recite the poem because its recital will mean something to you. If your rector wishes to understand that meaning, he should come to the recital, not set himself up as an artistic police officer. ;-)