That's why the idea of hybrids sacrificing effectiveness for versatility doesn't work out.
The idea that a hybrid class can be, say, a defender or a striker, but in exchange is not as good at either job as a dedicated defender or striker, will lead to them not having a place in the party. If they can't defend like a fighter, they aren't going to be wanted as defenders, and if they're a poor substitute for a rogue, people will prefer playing rogues. Even being able to switch roles mid-combat doesn't make up for being bad at both compared to pure classes.
The way you need to do it is as Baka no Hentai said. The "hybrid" aspect of the class needs to be reflected in class features you can take. If the druid is meant to be a defender/striker for example, they need to be made to choose which way they want to go through feats, class features, and the like. They should never, ever be able to change roles mid-combat.
This is one case where WoW offers a very, very good example (*gasp*). Previous to patch 2.0 or so, most of the "hybrid" classes in WoW were wanted only as healers. Druids had a tanking and physical damage talent tree and a spellcasting damage tree, paladins could build their talents around being able to heal, tank, or do damage, but the problem was that Blizzard had approached theese classes with the logic 3e jack of all trades were built around. In exchange for being able to fill any role in the game, they weren't as good as "pure" tanks or damage dealers. And, of course, nobody wanted paladins or druids that did damage or tanked, because other classes did those jobs much better.
Since then, Blizzard has changed their attitude, and (for example) tanking paladins and druids are equal to tanking warriors. To the point that paladins and druids have successfully tanked every high end raid boss in the game with a couple exceptions; some bosses have abilities that make a particular class impractical as a tank for it. They're all about equally good, with small advantages in specific situations.
Effectiveness being an acceptable trade off for flexibility is a fallacy, in other words. If a class is meant to be able to do a job, it must do that job as well as other classes that do it. And the choice of role to fill must be fairly definitive and changing it needs to be non-trivial or difficult. A druid being able to retrain their feats, unlearn powers, etc and switch roles would be alright, since they give up the old role to fill the new one. Being able to switch roles from fight to fight, or mid-combat, would be too strong.