Why do you and others feel they shouldn't be combined?
I can't speak for others, but the way I see it they are simply different paths, with different and somewhat conflicting goals at least at the, for lack of a better word, "student" level.
People often have a hard enough time attaining a constructive, functional understanding of any of them isolated, which is why having good, wise and dedicated religious teachers is such a urgent need.
Attempting to mix both at the same time, particularly
before a certain level of religious wisdom is reached, brings a significant risk of merging the weak points of both instead of complementing each other wisely.
For instance, one of the strongholds of Hinduism is its affinity and skill with the use of artistic inspiration, and one of its weaknesses is the reliance on the wisdom, caution and sincere intent of adherents. For Christianity, one main weakness is the nearly-complete reliance on belief in a Creator God that is supposed to be wise and caring despite not really showing it.
A mixed faith could inspire monotheists to realize what a loving all-powerful God is supposed to be by criterious use of well-chosen parables and artistic examples... or it could disastrously mix the arbitrary behavior of the Christian God with the arbitrary aspects and appearance of the various Deva to essentially rid itself of any clear meaning and become an exercise at using religious concepts to justify oneself no matter what.
That might in and of itself still be no worse than either path in isolation. But it would also be more difficult to understand and to correct by the example, examination and loving attention of others.
In essence, it would be taking the responsibility of creating a new religion out of available parts.