Given the ground covered in the past 2-3 posts on the idea of zecher l’mikdash, we can now begin to understand Ameimar’s opinion (Menachos 66) that counting weeks of sefira is unnecessary because the count is merely a zecher l’mikdash.
As we have seen, when a zecher l’mikdash is simply a symbolic act to remember what had been done in the past, the obligation to recite a bracha is not binding. The Ba’al haMaor (end of Pesachim) asks why there is no bracha of she’hechiyanu recited over sefira and answers that something which is only a zecher l’mikdash does not obligate a she’hechiyanu being recited – in the framework of the Brisker Rav’s analysis, the Ba’al haMaor is an indication that the mitzvah of sefira is less significant in terms of bracha than a true takanah.
And if the mitzvah is defined as a symbolic act alone, the tzuras hamitzva can change as well – non-shemurah matzah is acceptable b’dieved for koreich, pesulim of the 4 minim become acceptable after the first day, etc.
While Abayei understood (according to Tosfos) that the mitzvah of sefira is a Rabbinically mandated continuation of the same mitzvah of sefirah that had existed pre-churban and hence follows the same formula, Ameimar held of the more radical formulation of zecher l’mikdash – the original mitzvah of sefirah no longer applied, and all that we fulfill is a mere symbolic counting, which does not necessarily bind us to the same details in performance.