Never split the party. It’s one of the 4e mantras. Experienced players know that splitting the party often leads to disastrous consequences. D&D is a team game and as such the objectives almost always require a team to accomplish them. But just because the game is designed to be inclusive and keep everyone equally engaged, does that mean that there shouldn’t be opportunities for some players to split from the party and play to their strengths?
Sometimes circumstances will slightly favour one or two characters in the party. The most common example is to use monsters with vulnerabilities to energy attacks that the party has in its repertoire or have monsters attack using an energy types the party has resistance to. This may seem like a little thing but when it’s your character wielding the cold empowered bastard sword against the fire-based, cold vulnerable creatures it feels pretty great to have an advantage no one else in the party has. Likewise when your poison resistance lets you all but ignore the ongoing poison damage and shrug off a good portion of each hit from the poisonous serpents.
These kind of individual heroics are easy for DMs to place into encounters without throwing things out of balance and without excluding anyone (unless of course the entire party has fire resistance except for one poor soul). More importantly it doesn’t take any play-time away from other players. Everyone still gets their full normal turn, but in this kind of circumstance one PC may find that his turn is a little bit more exciting than his companion’s turns.