Purchasing Apple Mac Software: Options Available to the Consumer?
Is the Majority of Software available for the Mac now available for
  purchase from the Mac App Store ? Are there Pros &amp; Cons to Mac App
  Store purchasing?


I don't know exactly how you would compile those numbers, or if Apple even releases numbers for the Mac App Store, so it's difficult to say how the "majority" is sold.

As for pros and cons, probably the biggest pro is Mac App Store purchases can be used on any number of Macs that you own (see Apple's FAQ); you don't need to buy additional licenses for each machine as long as it's for personal use. Also, updates are delivered through the store, so you get a notification when updates are available.

Installing from the Mac App Store is also presumably more secure because Apple has tested and approved all apps before making them available (and can revoke an app developer's certificate, which is a benefit assuming a rogue app sneaks through, but could also be a negative if they abuse that ability).


  Do people still purchase directly from the company website that
  created the software as an electronic download - I understand most
  companies offer a free trial (usually 30 days)


Well, sure, why not? There's no requirement that all apps have to go through the Mac App Store. However, the Mac App Store does not offer trials, so you'll need to purchase directly from the developer if you want a trial (assuming they offer one).


  Do people still buy "boxed software" from retail outlets and online
  retail stores ?


Again, sure, of course you can if you want.


  If the software is available via all these routes and maybe purchased
  in other ways, is there a recommended (instinctive) way or do people
  just go through the above options and see which turns out to be the
  cheapest?


I'd say the Mac App Store is the easiest and most straightforward option for most users, since it's integrated right into OS X and updates are delivered automatically.