"Effectiveness" is subjective, so necessarily it will vary from group to group and from game to game. Still, as a DM, it sorta gets on my nerves.
A little bit of powergaming is okay, and even good (it shows that the player is interested in whether or not their character succeeds).
Character design also serves as an excellent cue to the DM as to how to design an adventure. You put in some elements that will play to the strengths of certain characters so they feel useful. Got a cleric or star pact warlock loaded up with radiant damage powers? Load up on some undead for them to blast. Got a warden? Give them a brute or soldier monster to square off with, and make sure it has condition effects that a save can end. Many times, the players will see things like that and think, "Wow, I'm glad I decided to play what I'm playing, I made a good choice." Or at least, "Oh cool, the DM put that in there for me to tackle."
Assuming that your players are actually interested in the success of their characters, a DM of even moderate skill can customize the play experience so that everyone feels like certain characters are useful. In fact, tailoring your adventures in such a way is required because players don't have foreknowledge of what you plan to do with the campaign. Sure, some players will try to make characters as useful as possible across as wide a spectrum of anticipated situations as possible. Some players will try to specialize to deal with specialized situations, usually either because of previous play experiences or because they anticipate that particular challenge.
In my own experience, characters are usually ineffective because of a miscommunication between players, or because they aren't well informed when they make their decisions.
Most DMs give sparse information upon which to design a character. "The new game is based in the Free City of Greyhawk. Your character can have any background you want, but give them a reason to start in Greyhawk." One player might decide to make a rogue who works with Greyhawk's thieves' guild, loaded up on urban-based skills like Bluff, Gather Information, and Knowledge (dungeoneering). Imagine his chagrin when the game uses Greyhawk as a home base and actually focuses on the intrigues of the Gnarley Rangers, or hunting and tracking orcs in the Cairn Hills. That player might have decided to play a ranger instead, had he been informed that actual play time in the city would be limited.