As travelers have adjusted to the carry-on liquid rules the overhead bins are back at full capacity. This definitely commends soft-sided, legal-or-smaller-sized bags:
On her US Airways flight from New Orleans to Washington in March, Corinne Marasco, a science writer from Kingstowne, Va., was forced to check her small wheelie bag at the gate because no room remained in the overhead bins.
It didn’t matter that the gate agents kept reminding passengers that they were limited to one item of carry-on luggage, plus one personal item; the overhead bins were stuffed with shopping bags, knapsacks and pieces of luggage that clearly didn’t pass the size test.
“I saw a woman with a roll-on suitcase, a medium-size tote bag and a pocketbook board the plane, and no one asked her to check the suitcase,” Marasco said. “I was sorely tempted to start emptying out the bin over my seat so I could fit my suitcase in. - NYT via Kansas City Star
The above story also suggests that the fees airines are levying for checked luggage is driving more inexperienced, rule-flouting flyers to carry on.