I was surprised too, since I had read that kind of thing here before I started DMing. But one solo monster is the worst case for Fireball -- it shines against multiple weaker monsters with lower hitpoints and saves. If you run an 8th-level adventure like
Cave of the Spiders, where most of the encounters are EL 8 built out of CR 4s, you'll see that 25 damage will take down almost every monster in the place. If they all fail their save, they all go down. All the encounters I planned in my earlier thread about monster roles ended up sucking because having several different weaker monsters let Fireball wipe out everything but the hill giant.
Ahh... that.
IMO, the problem you are seeing there isn't so much fireball as it is the ability of the CR/EL system to calculate the actual challenge involved.
The CR/EL system basically asserts that one CR X monster is equal in power to two monsters with CR X - 2. That's possibly valid when the CR is relatively high, or when CR X - 2 is approximately equal to the average character level, and in a few other narrow cases but it's not a valid assertion otherwise.
The fireball example you mention is one case of many. The basic problem is when the CR of the foe gets to be character level - 4 or greater, the ability of the creature to threaten the PC's starts decreasing exponentially. For example, according to the system, an encounter with 32 orcs is a EL 9 encounter and roughly equivalent in threat to a single CR 9 monster. This might be true if the average character level is 1, because 32 somewhat threatening attacks might equal 1 truly lethal one. But if the average character level is 9 then the orcs will be lucky to do more than scratch the party before being wiped out. For a 9th level party, the true threat represented by 32 orcs is closer to the threat represented by a single CR 5 monster. This is obviously a vastly different evaluation than rating them equivalent to a CR 9 monster, but in play is closer to what you experience.
Similarly, if you build a EL 8 challenge out of CR 4 monsters, the book will tell you that you should use 4. But if you do, in practice this is closer to an EL 6 challenge for an 8th level party; not a cake walk necessarily, but neither is it something you expect to consume alot of party resources. To build a balanced encounter for an 8th level party out of CR 4 monsters, you usually have to use more like 16, which the book rates as a ridiculous EL 12 encounter (signficant chance of a TPK) but in practice usually works pretty well.
You may have noticed here that the math I'm recommending is that 2 CR X monsters = 1 CR X + 1 monster. Keep in mind though that I'm only recommending this for monsters that are much lower level than the party and do not have level independent powers. If you do something like take non-humanoid CR 5 monsters and throw 16 of them at a level 8 party thinking that you are doing a EL 9 challenge, don't be surprised that it creates a TPK. As the monster CR gets closer to party level, the official math works better and better.
Also, in play you compare evocation to the other PCs, not to other wizard spells. It takes a full attack or a crit to match 25-35 damage to even one monster, much less four. I played two games at GenCon this year and both featured a round where a wizard Fireballed several targets for 50 damage total followed by a rogue shooting a shortbow for 1d6 + 1.
Well, rogues that aren't doing sneak attack damage usually are not combat monsters and 50 damage is unusually high for a 10d fireball unless its empowered or something. Generally speaking, at upper levels of play, if the rogue can't sneak attack, the rogue probably should be casting fireball or some such (via a wand of fireballs and sufficiently high UMD skill) or otherwise have a plan for dealing with the problem. I've seen a rogue in my game with rapid shot, high initiative, and a flaming shortbow that would have matched the fireball with something like 1d6+1d6+3d6+3 x2, which, as far as single target is concerned is as bad as getting hit by a fireball. In fact, I've had much bigger problems with archers having an easy answer for every tactical problem than I've had with evocation. If the wizard is capable of doing 50 damage with his fireballs on a regular basis, and the rogue isn't capable of doing something nearly as impressive it sounds to me more like bad contingency planning on the part of the rogue player as much as anything else.