Unfortunately for some people who are not overly computer literate remembering to change the Default Format that you save in is a different story.
Add to that they M$ change that Default Format with every Patch so if today you change 2007
I'm not sure about 2010 as I've never been required to change it but I can see no reason to believe it would be different to 2007 So if in 2007 you change Word to save as a .DOC File Format it works that way till the next Patch is released and then it changes back to .DOCX, not a problem if you are using GP then you can lock it in that position but some clients don't have a Network let alone a server and they do Government Tenders. The little ones up to 50Million $ Jobs and are not using computers all day every day let alone one program often enough to really get to know it. They are flat out remembering how to use the finer points of Quicken or MYOB Products which are used to pay their staff and they use those packages a lot more than Word.
I do however agree that M$ had to change the Word Format because it was nasty to work with and downright unstable so .DOCX is an improvement and shows just how badly Microsoft implemented Word to begin with. It was never in the same class as Word Perfect and only became the Default App because it was bundled in an Office Application where you could transfer Data between different Applications. That was a Brilliant Move on M$ Part as it saved multiple Data Entry Jobs in WP, Lotus and so on and is what made Larger Offices far more productive. But the entire basis of what Microsoft Office was built on is the real problem and things are going to need to change again in the not to distant future when they outgrow .DOCX

However on the other hand Word Perfect has not needed to change their Basic Document Format over it's entire production Life from the DOS Days to the current. It speaks volumes about a well designed product as apposed to a product that is Good Enough for now.
Col