to understand that basic statement is the fact the alot of people want to do something with the apple people have figure out it not good for the end product to have done to it ..
basically in laymen terms it the problem with jailbreaking it the whole thing now opens you up to security problems that they where trying not to have in the first place by keeping the system in a closed prostion to where outside applications have to go to apple first then get tested and then they are alllowed to be inside the apple product ..
To a point I agree with you. Having a closed system is a very large part of what keeps Apple products "safe", although if you follow those in the industry that actually analyze such issues iOS and even OS X are much less safe than Apple would have you believe. There is the possibility that jailbreaking will open you up to security issues, but I have yet to see a single case of that being true. To the contrary, the JB community has shown the existing security flaws that already exist in the operating system despite Apple's claims. The "jailbreak.me" exploit is a perfect example. This jailbreak would not have been possible if not for a gaping security flaw much like those that MS gets raked over the coals for on a regular basis.
This is a fundamental difference between "open" and "closed" systems. The only real open system I can think of that has a wide following is Linux. Because the source is truly open, any person can examine the code to find security flaws. In the end, this makes for a stronger and more secure system. Those flaws were certainly not obvious to the Linux coders much in the same way that the flaws exploited by the JB community were either missed or ignored by the Apple software engineers. As an iOS user (and OS X to a degree) I still believe this is why Android will in the end be a more robust and secure platform. The code is open, you are welcome to take a shot at bringing it down. I remember when Linux / BSD was a "mess" too. Somehow it became good enough to fork into OS X. It will be interesting to see where this all ends up years from now.
As far as "not good" for the device, take a look at what the main reasons for JB were. Folders, multi tasking, more file compatibility, etc. Seems like Apple has started to include many of the most popular jailbreak features into iOS. I sometimes wonder if all of those things would be here today if not for the JB community.