"Biggest"? The biggest problem with them is that there are so few paying customers in Cydia that devs simply won't develop any high-profile tweaks (DisplayOut, Quasar, Skype Recorder, RetinaPad, FirewallIP - just to name a few commercial tweaks really worth paying for) if they know they in no way will be compensated for their efforts / time because everyone is pirating their tweaks...
You are neither wrong nor right.
You can be successfull on Cydia. But one successfull programmer claimed he had 97% pirate rate for one of his tweaks. thats 97 pirated copies for every 3 payed copies. We will never know if he lied about the number, but he probably did not. Said programmer is a private person, and student, and lives of profits from Cydia sales very comfortably.
This developer is Aron Ash, the developer of Multifl0w and he spesifically states that pirated copies does not equate to lost sales.
its hard to know whats a lost sale,
http://www.cultofmac.com/193576/a-jailbreakers-perspective-on-the-pros-and-cons-of-developing-for-the-app-store-vs-cydia-jailbreakcon/
Things are not black and white, but you are not all wrong
I buy tweaks, I have pirated some too, a lot of which I have deleted and never used again because they were rubbish or I didn't want them. Some tweaks are Illegal in countries but not in my country, these are also in crack repos. And beta or alpha of some apps leak to crack repos to. And some legal tweaks are exclusive to some crack repos (idk why but only found there).
It is not my intent to promote crack repos here, my advice is to try to keep away from them. But misinformation is even worse. Its like drugs, even cocaine has medical uses (they do use it for instance to reduce bleeding in operations and more), ecstasy has been used as a psycho therapeutic drug in conjunction with therapy, but that does not mean that it is okay to have an infomercial on TV where a woman crushes an egg and says "This is your brain on marijuana" that's out right deceitful.
Sorry, I got a bit carried away, but I still stand for what I'm writing.