Dumb question: Why are you using a switching regulator AND a linear regulator?
The switcher gets it close to the desired voltage, then the linear cleans it up. Better efficiency than a linear alone, less ripple than a switcher alone. With an LDO you might drop your efficiency by a few percent, while reducing your output ripple by a couple orders of magnitude. Depending on the application this can be a good tradeoff.
True, but it depends on the power loss. I read if it's less than 0.5 watt, the extra cost for the switching reg doesn't justify the improvement.
Says who? Whoever is willing to make such a general statement like that doesn't know what they're talking about (or it was simply taken out of context). There are MANY applications where an extra dollar or two on the switching reg and associated components is peanuts compared to a 50+% reduction in power consumption with equivalent ripple. Wouldn't you rather spend a dollar more on your phone if it meant it could last 18 hours instead of 12 between charges? Who cares if it saved less than 0.5W if it made a marked improvement in power consumption? It's about percent improvement, not raw power numbers.
For any given application, you should make a table (even if only in your head), with columns for cost, board space, power consumption, and ripple. Your rows would be switcher alone, linear alone, switcher+linear, and maybe switcher+ldo (cutting it closer than with a regular linear reg). Compare the numbers and decide for yourself what the best approach is for that application.
Anywhere you have a big delta between the input voltage and the operating voltage, and ripple is of importance (ADCs, DACs, etc), the switcher+linear combo is going to make a really good case.