I'd take crosswind's bet too -- particularly since we've seen reports that you could somehow (we now have an idea of how) get the Fighter class feature -and- Sneak Attack.
Re "why do you get 19th level class features if you multiclass at 19th/20th level when the base class has to work their way up there?" Because getting much lower features would be worthless. The penalty you take for multiclassing is spending an extra resource (feats or paragon path) and that unless you're making a very careful build, the base class has much more synergy between their effects than you do.
Re multiclassing to striker: it seems like a very obvious and lucrative thing for -strikers- to do. (obviously, you can't multiclass to the same striker class twice) OTOH, I'd guess that the paragon path powers are better than the powers you get from multiclassing, and that the striker paragon paths generally also give you extra striker-type abilities; that's the point, after all. But re non-strikers multi-ing to striker -- that's far from a no-brainer, as "I do lots of damage to one foe" isn't what the non-striker roles are about. Sure, you -could- do that...or you could get much better oppy attacks (fighter), or some really good options for -mass- damage or controlling the battlefield (wizard), or a general improvement to your party buffs (leader), and let the strikers do their own job...
Re "why do you get 19th level class features if you multiclass at 19th/20th level when the base class has to work their way up there?" Because getting much lower features would be worthless. The penalty you take for multiclassing is spending an extra resource (feats or paragon path) and that unless you're making a very careful build, the base class has much more synergy between their effects than you do.
Re multiclassing to striker: it seems like a very obvious and lucrative thing for -strikers- to do. (obviously, you can't multiclass to the same striker class twice) OTOH, I'd guess that the paragon path powers are better than the powers you get from multiclassing, and that the striker paragon paths generally also give you extra striker-type abilities; that's the point, after all. But re non-strikers multi-ing to striker -- that's far from a no-brainer, as "I do lots of damage to one foe" isn't what the non-striker roles are about. Sure, you -could- do that...or you could get much better oppy attacks (fighter), or some really good options for -mass- damage or controlling the battlefield (wizard), or a general improvement to your party buffs (leader), and let the strikers do their own job...