R’ Akiva Eiger questions the common practice of the chazzan reciting a bracha on sefira, the tzibur answering amein and listening to his count, and then each member of the tzibur reciting his/her own bracha and counting – why should each individual recite his/her own bracha and not simply have in mind to be yotzei with the bracha of the chazzan, fulfilling b’rov am hadras melech? Although various achronim suggest reasons for the practice, the question seems to me to be far stronger than the answers offered (is it really so hard to concentrate on a short bracha while bearing in mind what night of the count it is?)
The same issue may be raised with respect to the bracha before k'rias hallel. The common practice is for the shat”z to recite the bracha, the tzibur answers amein, and then each individual again recites his/her own bracha. Especially on Rosh Chodesh where there is a major dispute in Roshonim whether a bracha should be recited at all, why not simply listen to the bracha of the shat”z having in mind to be yotzei and answer amein, accomplishing b’rov am hadras melech as well as minimizing the chance of bracha l’vatalah?
If I recall correctly, Rav Soloveitchik opposed each individual reciting his/her own bracha after hearing the shat”z (I recall hearing this with respect to hallel in particular) based on a different consideration. The Rav held that once one answers amein to the bracha of the shat”z, it is as if one has recited his/her own bracha because of the principle of shomea k’oneh and repeating the bracha would constitute a bracha l’vatalah. I believe I heard from my rebbe, R’ Y. Sacks, a proof to this position from Brachos 54. The gemara records that R’ Yehudah was sick; when he recovered and was visited by other Rabbis they praised Hashem for sparing his life and he answered amein – R’ Yehudah then remarked that by responding amein he fulfilled his obligation to recite birchas hagomel. The Shita Mekubetzet notes that those reciting the bracha clearly did not have in mind to be motzi R’ Yehudah – the “shomea k’oneh” which answering amein invoked seems completely independent of kavanah to be motzi or to be yotzei by that means. With respect to hallel, answering amein to the shat”z would similarly be the equivalent of reciting one’s own bracha even without specific kavanah. It was not clear to me in looking back at the Shita Mekubetzet whether the same holds true if one has specific kavanah not to invoke "shomea k’oneh" (IOW: does shomea k’oneh work as a mechanical principle outside any kavanah considerations, or does the shita mekubetzet simply mean there is a level of "stam" kavanah that is assumed sans specific intent to the contrary, but kavanah is still a necessary ingredient?). If one accepts this chiddush, it would seem that it is certainly preferable to listen to the shat”z and be yotzei with his bracha or to recite one’s own bracha word by word simultaneously with the shat”z’s rather than to repeat a new bracha after answering amein.