[pushkins]

When you use roofing nails you can run the same risk of floor movement, roofing nails as well as all nails can come loose (just like on your roof), that's why it is always recommended to use a mortar bed under the backer board when using nails. Screws bind tight and do not come loose over time.

It doesn't mean you will have problems, if you have a stable sub floor you will be fine, if you don't, then there is always the possibility of movement in the sub floor then transferring to the backer board and then you will see cracks in the grout and or tiles.

It's easy to tile quickly if you have to:
Lay the backer board without a mud bed, tile with quick set tile glue and grout 4 hours later.
or
Lay the backer board with quick set mortar bed wait 4 hours (or start tiling after you lay the last sheet), tile with quick set and grout 4 hours later.
or (as I've seen done)
Lay the backer board, glue the tiles down and start grouting the next morning.

Like all work, just because someone cuts a corner doesn't mean there "will" be a problem, it just means there is a much higher chance of it happening.
A bag of mortar to use under the backer board is $12 and for the average home owner an extra day in drying time isn't as bad as pulling up tiles to redo a project.

I do many major bathroom and kitchen renovations a year, I am cautious, I don't like unhappy clients and I have never been asked to repair any work we have done, a bag of quick set is about $15 and we can be grouting 4 hours later.