While primogeniture indeed wasn't the law, the first born did get more, and the others would have a right to plots which were _far_ below subsistence limit. The average woman from the norse age would give birth to about 7 children. No matter how you combine the inheritance from the guy and his wife's dowry, a plot would have to be divided among about 2-3 resulting families. (The other children would die earlier.) A plot which is already the size of bare subsistence for one family, is pretty much just that. No matter how you sub-divide it, a half or third of it is not going to feed a family.
Such plots had historically been created, especially among cattle herders, but they were simply non-viable.
Either one or more chose to go "a-viking" (raiding) instead, or they all had to, because each one's inherited land simply couldn't support any of them. Eventually someone would own a parcel the size that could feed a family again, one way or the other.
Raids were indeed a major source of slaves... ironically not just for the raiders. Being captured while trying to raid a settlement could turn one into a thrall just like that. (If you were lucky enough and they didn't execute you instead.)
I'll also return to the claim that not all were equal and there was already a rise of a knightly class. E.g., in Iceland, to be legally allowed to attend a Thing, a man had to be "able to ride a full-day's journey, and
bring in his own hobbled horse after baiting, and find his way by himself where the route is known to him." My emphasis there. (See, for example, here:
http://www.hurstwic.org/history/articles/society/text/laws.htm ) They were not all equal. Whoever couldn't come on his own horse or wasn't trained in riding, had no say whatsoever. You were only Thing-worthy if you owned a horse and were trained enough to ride for a day.
This is not much different from knighthood in continental Europe: the knights, although nicer formulated eventually and packed in a nice BS chivalry story, were essentially just the class who could afford a horse, to fight on horseback.
Also in Iceland, "
Although still free men, paupers and vagrants were classed even below freed slaves, in part because they had no residence, and thus could not be charged. The poor were not allowed to marry. The medieval Icelandic law book Grágás states that anyone was free to take the property of a vagrant without penalty [K131] and that it was lawful to castrate a vagrant, even if death ensued [K254]." (
http://www.hurstwic.org/history/articles/society/text/social_classes.htm )
(Maybe it's just because I'm a man, but the thought that anyone can come lop one's "family jewels" off for just not owning a house, makes me cringe. The thought that even cold blooded murder, if disguised as an accident while trying to castrate you, was perfectly legal too, even more so.)
Lacking a home also precluded one from appearing before a Thing, even to defend oneself when accused of some crime.
So being the younger son who inherited a tiny plot of land and no home, seems to me like the pressure was right there to go a-viking or become a mercenary instead. You didn't have all that much to look forward to at home.
Now the Grágás _is_ early 12'th century, but I find it a bit hard to believe that such ideas would be late imports from the rest of Europe. If nothing else, because I'm hard pressed to find a part of Europe where such laws might have come from.
It seems to me like you have your class divisions right there, even for Iceland.
Also, it seems to me like the Sparta comparison may actually have more merit than I thought. Leaving a newborn out to die of exposure, if judged unfit, seemed to have been so traditional that even Christianity and later laws couldn't end it. (See:
http://www.hurstwic.org/history/articles/daily_living/text/Demographics.htm or
http://www.voluspa.org/nornschildren.htm )