In practice this is what has happened round here for our LFR - everyone has several characters & every team ends up pretty balanced. It was not like this back in my Living Greyhawk days (4 wizards, a bard & a rogue being a particular low point, especially for the rogue).
Huh?
Having multiple arcanists made the best teams in 3e.
My LG team often consisted of 4+ arcanists (usually 3+ sorcs), and it was made of win.
Best game ever had 4 sorcs (full caster PrC), 1 wiz (full caster PrC), 1 wiz (arcane trickster).
With divination spells, timely pre-buffing, me tanking (I had 141 hp & 11-20 temp hp) and some ridiculous DPS (err, I mean damage per round) ...
Multiple teleport and/or dimension door capable characters = win.
But .. back on topic.
In 4e, some characters are simply better suited for ever-changing party composition than others.
Battle cleric? Yes please. Even if the party = 3 other leaders, it's a win.
Sword & board fighter? Can work well with just about anyone.
Of course, there are weird circumstances that will prove troublesome.
Being heavily armoured and finding yourself in a party of move 7 elves that like to kite could prove very painful.