I think you maybe be referring to 
's W "Rewind" since his E is the one that speeds up allies and slows down enemies, but either way you bring up a good point! Zilean is definitely one of the older champions in the game, and with the preseason after worlds just around the corner it can be a great opportunity to take a look at our Chronokeeper.
I agree with you about his "Rewind" ability not being a fulfilling ability to use. For me, it goes beyond that it's a skill you just spam, but one that completely lacks interaction with the enemy opponent. This is a quote is from the Dev Blog: The Design Values of League of Legends under the title "Meaningful Choices"
A meaningful choice requires tradeoffs. If it's a no-brainer, it's not interesting.
Which is exactly how I would describe his W ability: a no-brainer. As you said, you just spam it to get the CD's off. Whether or not the spell should (or can) be interactive with the enemy is up to the design team to figure out, but adding that element might make the ability more interesting.
However, his W "Rewind" isn't the only thing that should be looked at. For example his passive, which increases experience gain of all allied champions by 8%, is what Riot would consider "hidden power". In case you weren't around, Riot has been trying to phase out "hidden power" from the game by eliminating/reducing the effects of global aura's that Janna's and Twisted Fate's old passives use to have. [Morello explains the thought process behind those changes in this thread from over a year ago if you're interested.]
In that thread, Morello basically states:
Global passives that provide free stats can't influence the game past champion select other than "the other team has extra stats." Something like "players near Janna move faster" would at least have a positional requirement.
So, it's clear that Zilean's current passive goes against with Riot's direction in their design philosophy.
So why haven't we heard about Zilean being reworked? Well, for one thing, it's probably not that high Riot's priority list. In terms of gameplay, Zilean is not overpowered by any means--its just that his kit is out-dated like some other champions 
and it just comes down to priority
Now, I have noticed Zilean being played more in the competitive scene, most notably in the NA LCS Playoffs. So if Zilean does somehow become a problem at the competitive level from all regions then perhaps Zilean might be pushed higher on that list ;)
((Hopefully a Riot member from the design team might see this and shed some new light or news. I summon @Riot, @Morello, @Ghostcrawler, @anyonefromthedesignteam ))
technically gives your team a double hyper carry.
