In the classical italian-scheme, which was not really a flat 4-4-2 system, the left-back had an important role to fill as a part of the offensive play. The classical rightback (like Bergomi) was a tough-takling marker who played almost like a central defender, and this gave room for a left defender who played as a wing-back.
Maldini was a world-class player, but people seem to forget that Cabrini and Facchetti was just as good, if even better.
As for Roberto Carlos, I don`t see how a key player of a team which have won two ligas and three champions league trophies during his time in the club can be overrated. Give him a few more years and he will go past Camacho as the greatest leftback in the history of the club. I can understand how people are disappointed that he don`t score more spectacular goals (like against Tenerife or France), but they don`t appreciate the true value of Roberto Carlos. Over the years, he has improved a lot when it comes to defending and his stamina is just incredible. Not to mention the deadly passes or throw-ins.
I think Maldini was a more complete player than Carlos, even though the latter is more spectacular. Also, Maldini dominated his side for a decade and is a very intelligent and fair player. But the difference is not that great, and more importantly, R carlos suits Madrids style better than Maldini would have. In other words, it`s a matter of playing-style. In an all-out attacking team like Real Madrid, there is no better than the small brazilian.