If your media is fine enough to infiltrate between the rocks they won't offer any improvement in drainage long term, unless you put some sort of filter in between the rocks and the finer stuff above. I've found the cheap synthetic cleaning sheets do a pretty good job of it.
If you're media is coarse enough so that it doesn't fill in the gaps between the rocks I don't know if it will improve drainage, though water retention might be affected.
I'll put rocks in the bottom of pots that I expect might have a tendency to get tipped over, be it from wind or just a top heavy plant. A heavy base helps counteract that.
But in the case of epiphytic orchids I think one thing that get's confused is wet vs. lacking oxygen. I've seen a lot of orchids that supposedly can't handle wet feet remain perfectly healthy with roots that never dry out for months at a time when growing mounted outside. Rainy seasons here can sometimes be a bummer with months going by without a single day where it's doesn't rain. I think the issue with wet roots is more of an issue of roots that aren't getting any oxygen. When the water is a thin film over the roots and moss, it can diffuse through it readily. But enclosed in a pot that's a different matter, especially if you have a substrate that is decomposing and using up the oxygen. So you may want to make sure your pot isn't sitting flat upon a surface that doesn't allow for air exchange from the bottom of the pot. Lots of pots have little gaps in the lower rim to allow for this. But I've also seen pots that sit in saucers that pretty much form an airtight seal as soon as any moisture gets in between them. I sort of want to try putting small perforated pipes vertically in pots with more water retaining media just to see if the roots gravitate towards them.
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