So many directors have said in the past that special effects sequences are best used in service to the story...

In principle, I agree.  At the same time, practically, that statement is so flexible in its meaning that it's more easily used as an excuse for excessive reliance on effects than as a measure for keeping films trim.

Before our current renaissance in effects technology, filmmakers relied on the performance of actors to "sell" a world or a situation.  As audience members, we've come to think of performances like this either as great, or as overly campy.

Today, it seems like many old school blockbuster directors are going for wider angles because they can.  Many of them feel liberated from their actors, because the visuals no longer need to be sold.  Effects are more or less at a point of perfection, where anything can be made to look real.  Those reaction shots and expository bits are no longer necessary to sell a world.  Now, you can just show it.  Do I agree?  Absolutely.  Actors no longer need to sell effects.

What these guys are missing is that selling the world was never the point.  When actors are working for effects shots, the outcome is cheese.

In my mind, an effects shot is necessary only as long as it backs up what the actor is doing.  Watching Neo say "Whoa" is the point of showing Morpheus jumping rooftops - Neo's reaction (and his growth) demand a whoa-worthy sight!  Watching an actor bear witness and interact is the point of spectacle in film.  We only see the sights for ourselves so we can believe the actor's reaction is real. 

That's what worked about Iron Man.  Every effect in that film was working for the actors - not the story mind you, the actors!  When they watched the sky, or their computer screens, or criticized each other, or stole plans or what have you, we believed it was merited because of what we had seen for ourselves.  Anything more than that in the effects and spectacle department is too much.

Can the same be said for the Matrix sequels or the Star Wars prequels?  Can it be said for Sky Captain?  In some cases, it's not an easy question.

What about Lord of the Rings?  To me, what I was seeing on the screen did the work of justifying Frodo's wide eyes, Gandalf's fear for humanity, and the courage of the Fellowship.   Actions were epic in LOTR, so epic visuals were needed to support them.  Sure, those effects were in service to the story, but so are most effects.  Even schlocky effects are usually used for plot.  These were good because they were working for the cast.  That's why I loved use of effects in the first two Spider-man films, come to think of it.  Why is Peter Parker so excited?  Watch and see!

Actors don't sell effects.  Effects sell actors - especially in circumstances where the actor can't do it alone.  That's the only function of a special effect.

Publicity documentarians love interviewing stars about tricky effects shots.  Listen to what they say about how the director got them through it.  On the really good films, the effects were there for the actors - and the director knew it.