So after researching a bit - I came across this from Rabbi Aryeh Lebowitz here.
It would seem that Rav Shlomo Zalman Auerbach zt"l raises my question. It brings in the sefer Siach Halocho, siman 6, se'if koton 4 that Rav Shlomo Zalman draws the conclusion that the nafka minah like @AvrohomYitzchok says, is whether one can be motzi his friend.
There appear to be two diametrically opposed proofs.
In Mishnah Berurah 6:10 it says that if a shaliach tzibbur starts bircas hashachar and then he realises that he has already said it at home (including saying asher yatzar) and he hasn't been to the bathroom since - he can get someone else to say asher yatzer on his and the tzibbur's behalf. However the notion of having someone else saying for another person does not work for bircas hane'enin, e.g. if a person wants to eat an apple and his friend doesn't. That friend cannot say the borei pri ha'etz on the first person's behalf. However, with bircas hashevach this would work and so this would lead us to believe that Asher Yatzar is indeed a bircas hashevach.
However, proof 1 would only ring true for the first asher yatzar of the day, but what about the subsequent ones - does it then turn into bircas hane'enin?
PROOF 2: If a person didn't say asher yatzar and then when he remembers he then need to go again, the halacha is that you don't say a new bracha. If we say it is a bircas hashevach then this doesn't work as in the same way one thanks Hashem once, he then thanks Hashem again. But if it is a bircas hane'enin then the benefit went away because the person needs to go to the bathroom again, and therefore the one bircas hane'enin is going to be motzi both of them.
So it would seem that it is not clear cut.