mitzvos lav l'henos and the brachos on matzah

The gemara (Nedarim 15b) writes that if a person takes a neder not to have hanaah from relations, his wife becomes prohibited to him.  The Rishonim ask why this is true.  We have a principle that mitzvos lav l'henos nitnu, and when a person is with their wife it is a kiyum mitzvah of onah, so where is the prohibited hanaah here?  Ran answers that mitzvos lav l'henos nitnu just means that the schar of the mitzvah is not considered hanaah, but certainly if there is physical pleasure associated with a mitzvah it is considered hanaah.  For example, the gemara (R"H 28) writes that a person who took a neder not to bathe in a spring cannot go to mikveh when it is hot outside.  Even though the person may be going to mikveh l'shem mitzvah, the physical pleasure that he gets is still hanaah that is prohibited.  Rashba gives a different answer: the mitzvah of onah is part of the shibudim obligations of a husband to a wife.  Since the neder breaks this shibud, m'meila there is no mitzvah.  (It is a bit hard to understand why exactly the neder breaks the shibud).  

Achronim (e.g. Avnei Miluim 28:60) assume that Rashba avoids Ran's answer because Rashba interprets mitvos lav l'henos very broadly as allowing any and all hanaah that comes as part and parcel of doing a mitzvah.  

We discussed this machlokes a long time ago, but since it is close to Pesach, here is the Maharatz Chiyus' hesber based on Tos (Pesachim 46a): Tos writes that point under debate in the machlokes whether nedarim/nedavos can be offered on Y"T is whether the heter to eat meat from the korban is just an incidental side benefit and ignored, or whether it allows the hakrava to fall under the umbrella of ochel nefesh.  So too here, the Rashba holds that physical benefit that results from a kiyum mitzvah is just incidental to the mitzvah act and therefore ignored, while Ran disagrees.  (There are many other hesbeirim, e.g. the machlokes Rashba/Ran is whether you say psik reisha by issurei hanaha -- the physical hanaah is a psik reisha of the maaseh mitzvah.)

If you are still with me, based on this Rashba that says even physical hanaah from a kiyum mitzvah does not count as enjoyment, R' Hutner asked: why do we need to say a birchas ha'nehenin when we eat matzah?  Mitzvos lav l'henos and the enjoyment of the food [you mean some people enjoy eating matzah?] counts for nothing, it's just a maaseh mitzvah, so just say a bracha of al achilas matzah and that's it?!

(R' Michael Shiloni has a long arichus on this in his sefer Maarachei Moed, but I'll leave it for you to enjoy thinking about.  There are many ways out of the problem...)