I never played 2e - I got into RPGs literally a week or three before 3e came out. I have not yet played 4e (nor am I all that certain I want to).
I answered with about 50 - 75% were not pure core classes. When I chose a class it was initially core about 25% of the time, but more often than not it would gain a PrC or multiclass into a non-core base class by level 8 - 12. Usually the core classes just could not cover the basic feel for the type of character I was striving to create.
The only class that ever came close - or, perhaps more accurately, was most often used without PrCs or multiclassing with a non-core base class - was the rogue. Its several diverse class specials combined with high class skill list and numerous skill points often allowed it to fill several possible roles from thief to trickster, from scout to trapmaster, and so forth. I might dip for 1-3 levels into sorcerer, bard, cleric, fighter, or nearly any other core base class to expand its options a bit, but for the most part it worked quite well even on its own. The same cannot be said of, say, the fighter. The lack of interesting higher level bonus feats for the fighter (until PHB2) really stiffled the playing of that class for many. Almost every time I saw someone play one (and the few times I myself tried the class), multiclassing with another class (often after 4th or 6th level) or adding on a PrC occurred. Only once did I see a person take the Fighter into high levels without multiclassing (level 11 or 12 before the character died, a half-orc fighter).
Casters were often played almost purely, but even then there were several PrCs available that diminished casting only slightly (no more than 3 caster levels) and so were still desirable for one seeking to reach 9th level spells (even if the game was unlikely to last to that high a level). The lack of significant class specials for the cleric and sorcerer (and, to a lesser extent, the wizard) made multiclassing or taking a PrC - when it was not too costly in caster levels - all but certain at some point, if only for flavor. I know of several instances where a PrC was taken that reduced final (20th level) caster level by 1-3 levels and gave moderate to lack-luster class specials were taken specifically more for flavor than power.
I still find it amusing that the PrC was initially created in 3e DMG as a type of 'kit' for those joining an organization, meant to be infrequently taken and controlled rather strongly by the DM, but instead they became one of the most commonly accepted practices in the game and often were not associated with role play requiring organizations (that one would have to petition to join, perhaps having duties or such after joining).