I have found that this tends to be a heated debate among some of the Wiccan and Pagan community, even though mixing Pantheons is a common practice in Wicca. I can see where the opposition is coming from in the sense that they believe it to be disrespectful to use bits and pieces from religions and not use all of it. But what they fail to realize is that no one follows every single thing of any religion. For example, is a Christian honestly going to try to follow parts of the Bible where it says to kill anyone who works on Sunday or who believe in different Gods? I think not. Are all of the women preachers going to follow I Corinthians 14:34-35? Obviously not. So we all pick and choose what works for us and what does not. In Wicca, we know that what may work for one might not work at all for another. And so, I think that we should draw from practices and beliefs in religions that connect with us and that we can work well with. I have mixed Pantheons, and I have not felt any bad energy from the Gods for doing so.
In Wicca, there is a belief in a God and Goddess. But the reason there are no specific names for those deities is because God and Goddess is different to each individual. To me, Goddess may be Mary. To you, Goddess might be Athena. So it's what or whoever you believe to be God & Goddess. I have yet to see a spellbook or any Wiccan or Witchcraft book that does not mix Pantheons. I was reading a spellbook recently which taught how to dispel a Ghost, and it suggested calling on Anubis and Hermes. These are different Gods from different Pantheons. And these are acceptable practices in the Wiccan community.
It is also very difficult to find a religion that did not borrow from another one anyway. The best example I can give is Christianity. We Wiccans & Pagans know that Christianity would have no foundation had it not drawn so much from Pagan cultures. As an example, the eating of bread and the drinking of wine to symbolize the body and blood of Christ was originally an Egyptian Pagan practice where they ate wheat bread and drank beer to symbolize the same of Osiris. And I am sure there are other Wiccans and Pagans who can tell me of Pagan religions that borrowed from other Pagans.
Another perfect example, I think of mixing Pantheons in the Wiccan community is the Goddess chant. It calls out to Goddesses from many different Pantheons. Some believe that the Gods and Goddess have always been the same deities, just with different names. For example, some believe that Artemis and Diana are the same Goddess, just with different names, and I also believe they are one in the same. I think that no matter which name we choose, we are calling to the Gods and Goddesses. What do you think?