True -- occassionally those guys were capable of ad libbing, while they knew ultimately the priority roles of playing QB and winning meant:
1) reading defense alignments
2)adapting and adjusting to said defensive alignments
3) passing with efficiency
4) communicating with your own receivers
5) knowing when hold 'em and when to fold 'em. (something Favre never quite learned himself)
Ad libbing, scrambling, and "making things happen" off of broken plays is rarely the rule against well-prepared defenses this deep into the playoffs -- as McNabb has now proven three straight years.
That has an awful lot to do with these losses. I'd say an additional factor is the lack of physical receivers on the Eagles' team - good defenses in the playoffs are jamming them at the line, and these receivers can't get into their normal route as a result.
(Btw, I forgot Montana and Steve Young on my list. .....and Hostettler could scramble a little too. But the vast majority of Super Bowl-winning QB's were pocket passers).