Your reasoning is still a little twisted. Let's straighten a few things out.
An advantage of the Fighter's Combat Challenge is that it doesn't take an action. Aegis is a minor action. Usually spending this is not an issue, there's not a whole lot of other interesting actions vying for the minor slot.
The "when powers allow him to do so" is a non sequitur if I ever saw one. Yeah, a Swordmage can only mark someone who's within 2 squares, and only if he has a minor action available. Well, duh. A Fighter can only mark someone he's attacked. Otherwise he's not allowed to do so.
As for "any time"... well, any time he makes an attack, sure, but not any time he feels like it. This might actually be more limited then Aegis in some cases: if a Fighter has used his standard action for the round on something other than an attack, then no marking for him.
And "as many people as they attack" is only true if you mean as many enemies as they make an attack roll against during a turn. Multiple marking is dependent on using powers that have multiple targets. In other words, encounter or daily powers. So keeping a bunch of people marked indefinitely is not a basic class feature.