I'm not sure I entirely follow what you're saying, but I'll give it a shot....
To continue the analogy, those two extra arms are stronger than your regular arms, but ill-suited (due to placement, lack of thumbs, actually being tentacles, something) to wielding weapons. They're great for punching, but you need to adapt any fight style to use them well. Assuming you need to know how to fight (adventurer), you have the option of learning conventional styles (wizard) that put you on even ground as any average Joe. Or, you can play to your strengths and figure out how to use your super-cool tentacles. If you split your learning, you won't be as good with either, but you certainly could. Focusing on your tentacles is single-classing in sorcerer. Focusing on traditional style is wizard (or fighter, even). Multi-classing is pretty self-explanitory.
Or, to use a more personal example, I'm a pretty big guy (bench around 250#, 300# when I was younger) and, since that's my "natural" physique, it doesn't appreciably slow me down. I've picked up some martial arts training, over the years. While I'm not going to jump into a pro MMA ring, I'm confident in my ability to stand up to a couple of "average" guys, should they want to go for it. I also have my concealed carry permit and don't suck with a handgun. I don't generally carry, though, and rarely go to the range. Why? In 99% of real-world situations, including someone breaking into my house, the handgun is going to be less effective for me than my fists. You can disagree with my opinion and/or assessment of my abilities, if you want. The fact is that a sorcerer is much more likely to think the same way, though. Why would I rely on an external mechanism that's inferior to what I know? I've got a good friend who's a natural marksman who would play to the opposite end, though. Since we're using a metaphoric argument, the same basic train of logic applies.