You're asking why someone would prefer to join a group of mercenary vikings instead of sitting around a roman court waiting for his father to die?
I'm sorry, as a Swede I might be biased but.. I just don't see the "why" in this : P
Haha. Touché.
I think it should be possible to assign your sons to holy orders, the church and mercenary groups where it make sense in the realm or region you're in. The Genoa Navy would recruit leaders from Genoa's court (like from the Doria family) etc.