If you are not paying attention - which most people don't - you'll be installing unwanted software, because so much relies on basically crappy software (Flash and Java). Actually, that makes them crappy is their ubiquitousness. Even if you seldom use Flash or Java applets, you'll often find yourself having them installed, because there is that one site that needs one or the other.
Now, people like us with the technical knowhow know how to get around that (disable plugins; flashblock, etc.), but most people just install these things whenever there are updates, each is a change of getting crapware along with them. And Java is quite often updated.