If you ask most Vagabond fans who their least-favorite character is, most will say Matahachi. Not only does he seem to be a distraction from the main story arc, he's a cad and a fake – leeching off the reputation of another just to get by with a pathetic existence.
However, I think many readers miss the point of all his embarrassing subplots. Matahachi isn't just there for comic relief. Inoue focuses on him for a purpose, and I believe it is to show off a kind of dark mirror for Musashi's ambitions. The path Musashi followed, which Matahachi envies so much that he later emulates, ends up corrupting his entire life, in the same way that it nourishes Musashi's.
I believe Matahachi is Inoue's (and by the same token Eiji Yoshikawa's) way of commenting on the pitfalls of this era in Japan for opportunists who sought the quick road to glory in the years after the war. It's only through the lens of Matahachi that we this aspect of the age – the power and influence of the “ascending dragons” phenomena.
For every true hero like Miyamoto Musashi and Sasaki Kojiro there countless others who turned their brief reputations as swordsmen into careers by seizing the opportunity of the era. (Musashi and Kojiro do later “cash in” their reputations for careers to some extent, but Musashi manages to maintain his credibility, and it's well after his days as a vagabond). While these men were surely revered and respected, to outsiders it must have appeared like they put themselves on the fast-track to success. To Matahachi, this was his ticket to glory, and for a time, he always felt like it was just another year of training away for him to achieve it ("I'll make a name for myself, here in Osaka! You'll see, Takezo!"). But it's a dangerous journey, fit only for few, and he rode it all the way to spiritual bankruptcy.
Musashi was able to survive and become a legend without compromising his path because his extraordinary abilities and spirit are suited for such a life. But consider how many lesser men must have fallen off that impossible mountain during the climb to the top. And I don't just mean the physical and fatal consequences – that aspect is well-covered by Musashi's story – but also the sociological and psychological impacts. This was a cultural phenomenon that was quite literally sweeping the nation. Consider how enormously popular the “way of the sword,” was at the time. Takuan says in volume 11 that the samurai practicing Zen meditation outnumbered the monks in his temple. Musashi led the charge in this era on a path that must have cut a swathe of broken, lesser human beings across the entire country. His childhood friend just happens to be one of these pitiful creatures that takes center stage so we can see all the gritty details of those in the gutter of his lifestyle.
So for those who would write Matahachi off as just an annoyance, remember that Vagabond isn't just a story about Musashi, it's also about the time in which he lived, and the people that inhabited the world around him. Matahachi, far more than Musashi, is truly a man of the times.