Why? Because for Shanahan and for owner Dan Snyder, McNabb is essentially a caretaker quarterback. He's someone who gives Washington's suspect offense an air of legitimacy, someone who gives the team at least a fighting chance, and someone who brings a nice-guy image to the face of a crumby franchise that hopes to keep fans paying for overpriced seats as the team rebuilds.
This team has so many holes to fill, on offense and defense, it cannot necessarily afford to invest immediately in a new franchise quarterback. And even if it wanted to, it could take several years to find the right one. And there should be no hurry sign one anyway, it also could take several years for the team to recruit the personnel needed for a serious run at the playoffs. Redskins fans by and large recognize the team is in desperate need of a major roster upgrade. But at the same time, they're impatient for competent play after years of mediocrity despite front office hype and flopping big name acquisitions. McNabb at least theoretically brings competence and credibility at that one key position as the team endeavors to upgrade its O-Line, receiving corps, D-Line, and so on. He doesn't have to be great. If he plays well across the season, he'll keep hope alive among the fanbase and butts in seats at Fedex Field and he'll make good money. And if his play is poor or the team is able to bring in a better replacement, they reportedly could cut him without a massive financial hit.
It seems they got this one right. But maybe not. An irritated and impatient fanbase, inflamed by sometimes rabid sports talk and DC's notoriously aggressive sports press, simply may not tolerate even a competent team, as the team rebuilds, and may drive him out of town.