When I think of that phrase in the context of how it is used in the temple, it is important to remember that it is used to describe people trying to gain a following - and that it originally was focused specifically on "preachers". In its purest form, the primary focus is teaching the philosophies of men - meaning, to me, that someone is teaching what they see as man's ideas void of any reference to God. In other words, at its base it's a structure that denies God. That fits its use by Lucifer as something he taught and of which he approved. That, however, would not appeal to religious people, so the secondary focus is scripture - and I see "mingling" as "adding just enough to get people to buy into the philosophies of men". At its most manipulative, it is using just enough religion intentionally to make religiously oriented people think they are focusing on God when they actually aren't.
In other words, the combination of the two, worded in that way in the temple context, means that someone is crafting something intentionally that contains just enough "God-speak" to deceive people who otherwise would not believe them - and it focuses on what is taught (the philosophies of men), not what is left out (a comprehensive scriptural foundation) in an attempt to convince the religious. Again, it is just enough religious justification to make it sound religiously valid. That is critical, in my opinion, when trying to understand some people who are trying to convince religious people to follow them.
There are lots and lots of people who fit that description - both in religion and outside of it. It's rampant in politics, for example, where religious terminology is used to appeal to religious people. There are problems with the other extreme (teaching scripture exclusively, believing it is the inerrant word of God and not realizing it also represents, in some cases, the philosophies of the men who wrote or compiled it - or, in some cases, was understood originally to be figurative, allegorical, mythological, etc.) or teaching exclusively the scriptures and condemning entirely the non-canonical philosophies of men (with advances in scientific understanding being the best example), but, for those who are "preachers of religion", focusing on human, non-God-centered wisdom first and supplementing only as much as necessary with God-focused,"faithful" teachings is one of the purest forms of heresy.
People can argue constantly about the exact nature of specific teachings, but I believe it is the orientation and focus that is the point of the temple phrasing.
Finally, I think that phrase is used as a hammer in many situations where a hammer isn't necessary or appropriate at all - generally to mean:
"That's not how I interpret that scriptural passage, so you must be relying on the philosophies of men."
Having said that, I also believe the tendency to teach the philosophies of men, mingled with scripture, is natural to all who are religious - and I think the Priesthood ban is probably the best example in Mormon history of using scripture to justify the philosophies of men. Members of the LDS Church absolutely aren't immune to the principle involved, since all of us tend to see what we believe in the scriptures. Just as "nearly all" who have a little authority, as they suppose, tend toward unrighteous dominion, nearly all tend toward mingling scripture with human philosophies (using scriptures to explain our natural beliefs). That is very much part of our natural selves, and I believe it's important to understand that tendency.