one reason one might not do so is that the blessings of shema were instituted by the rabbis and with a safek d'rabanin one is often linient against a safek d'orysa (bracha levatala)
one reason one might do so is that blessings themselves are connected with the fulfilling of the mitzvah in the most ideal way.
If one is in doubt whether they had already recited maariv should they say the blessings of shema with the shema? If not what should the order of maariv look like exactly?