Mexican poetry, Peruvian poetry, Spanish poetry, Cuban poetry, is all written in the same language. If I gave an educated Spanish speaker a poem written by a Uruguayan, an Argentine, and a Spaniard, sh/e would not be able to identify the nationality of the authors, unless the poems happened to have geographical markers particular to a region or nation. Most poems have nothing of the sort. A lot of contemporary Spanish American poets view poetry written in the Spanish language as a single, unitary tradition.
On the other hand, Latin Americanists tend to specialize in nations and regions. You can be a Southern Cone specialist, a Carribeanist, an Andeanist, a Mexicanist. It makes sense to study particular clusters of poets geographically. And, of course, the division between Peninsular and Latin American studies is strong in the academy.
So we have two ways of dividing up the field: by language and genre, or by geography. With poetry it makes more sense to look at language and its particular traditions, in my opinion. This is borne out by the amount of transatlantic movement and influence, and intracontinental interchanges.
It occurs to me that one measure of this unity might be the number of scholars who write about both Spanish and Spanish American poetry. If I write about both, then I will myself be bolstering the argument for unity, by being one more example of this phenomenon.