A good DM should focus the game on party roles so that class balance isn't an issue. Each party member should get their opportunity to contribute and shine. However, poor DMs just throw out random combat or blindly follow a scripted adventure, where support characters may get lost in translation.
My first experience at 2E D&D, I created a Rouge/Thief because the party needed one. (My first PC ever was also a chick which weirded out the group, but I assumed the purpose of roleplay was to play something foreign to your mundane life.) All we did was instanced combat from random encounter tables. "From the shrubbery leaps two Storm Giants! Roll dice!"
The Fighters and Clerics were better suited for melee, and healing. My Thief was not as suited to combat, and I also failed to receive the bonus 10% experience for combat. Because the DM didn't go out of his way to include my PC more directly, I was really bored. I asked to create a Fighter and throw away my PC, but the DM refused until my PC died. After that I tried to kill off my PC by acting in suicidal fashion.
The PC never died, and thus began my streak of improbable surviving PCs, and much of my playing style stemmed from that.
We played Dragonlance at the time, and we were in the middle of a Dragon War. Blue Dragons attacked our castle, and I leaped from the top tower on the back of a Dragon, attempting to backstab it mid-air. I succeeded, and after finishing off the Dragon, I fell 200 feet to the ground.
20D6 damage, and a successful saving throw later, I still survived. It was lots and lots of that.
I made 23rd Level Rouge, 10th Level Psionicist before I finally retired the character.