In my game there are 5 PCs. One is hybrid (ranger-cleric) and 3 are multi-classed: wizard/invoker, fighter/cleric, and sorcerer/monk (he wanted Thievery training and likes the no-shift lockdown from his Flurry of Blows).To me it seems like a good idea to just have a bit of diversity in your character, but I could be totally wrong. I'm very new.
I would advise a new player not to go hybrid - it can be a bit of a trap if you're not sure what you're doing.
But multi-classing isn't a trap - you get training in a skill, which makes you more versatile, and you get a nifty extra ability. Multi-classing as a leader (cleric, warlord or bard) can be escpecially nifty, as it will probably open up a skill option that isn't on the rogue list, and you get to pack your own 1x/day heal, which I think might be useful for a rogue - you're pretty squishy, and if you're planning to be mobile you might easily get more than 5 squares away from the party leader.
And if you decide your multi-classing doesn't work out, you can always retrain the feat. The wizard in my party was a wizard/cleric before he retrained to invoker (to get access to a paragon path), the fighter was a fighter/warlord before he retrained to cleric (to get access to a paragon path) and the sorcerer was a bard, and then a cutthroat (from Dragon magazine - he wanted Stealth plus Bluff as a minor action) before retraining a second time to monk.
At least that's my take on it.