You most definitely don't need her. She's useful at times, but... you don't need any character. (And yeah, that includes Wynne.) If you like Morrigan more, then go with Mor.
Unless you have the hotfixed installed, she (as an archer) will not really compare to a dual-wielding rogue damage-wise. If you grab her early enough (IE, no hanging around leveling yourself up with the Duncan blood trick), though, you can just start moving her up the dual-wield track instead, though... or you can go talk with a Raven.
Trap disarming, lockpicking, and pickpocketing are all useful- the first admittedly mostly for XP (and to save your silly warriors from some stuns, occasionally) and the third for some loot. Lockpicking is better then the other two: there is a ton of XP to be made picking locks (someone who picks every lock they come across versus someone who never does will end up at least a level higher), and there is loot. The best loot isn't usually found in a pickable chest, but often an item to complete a set is, or just some money, and perhaps worse, some very nice gifts. These gifts are never the 'stop the whole party to squee over them', but quite a few 10 point gifts are locked up here and there. However, in most areas, nothing stops you from running through with your usual party, and re-jogging through the area with Lei later.
If you grab Zev early enough (or respec him), he can act as your groups lockpicker. Or, of course, your main character can, if you're a rogue. All in all, I think Lei isn't required at all... but a rogue of some kind does help. Who that rogue is should be up to you.