I have seen so many episodes of Pokemon. Many episodes have Team Rocket trying to steal Ash's Pokemon by disguising themselves, or with other predicable methods. Still, they are easily recognizable as Team Rocket every time. Why are Ash and his friends fooled every time, when Team Rocket's actions should be predictable by now?
It's not that Ash and co. are constantly fooled by Team Rocket again and again, but rather that Team Rocket has a specific role in the show. In order to keep Team Rocket in the show, they have to have a purpose for each episode, and thus have been constantly after Ash's Pikachu since the original few episodes. To us, it seems that Ash is "dumb" or oblivious to Team Rocket's schemes, but in all reality it is due to the writing staff trying to keep them in the show (you have to remember that Pokemon is considered to be a children's show).
I found from here one reason for why Team Rocket still appears in Pokemon:
Because they are popular comic relief characters, and if they were ever to get rid of them, a boatload of rocket shipping fangirls would freak out and hate Nintendo and stop buying games and watching shows, therefore bankrupting the company. How else would they manage to make those episodes last 22 minutes without wasting our time with another predictable plot to capture Pikachu?
At this point, Team Rocket has become a staple of the Pokemon universe, and it would feel empty without them. There are other Pokemon watchers who feel similarly to you, as stated here:
The comic relief they provide is a necessary part of the show, but I'm quite certain they could think of better ways of working it in. Not quite the same as how you feel about strangling them, but my brother has actually started cheering them on because he wants them to actually win for once so that they aren't completely irrelevant. Plus it would be fun to see Ash totally lose it when Pikachu is gone for days on end ;) The thing that really bugs me though is how they have no concept of how you're supposed to pull off crimes anyway. I can think of several instances off the top of my head where if they'd kept their traps shut (no pun intended) without constantly going off into their motto that they might actually have succeeded. They also think that as soon as they've got a net around something that it's theirs. Team Rocket is always shocked when the Pokémon fight back. Finally, who the **** travels in a hot-air balloon when you're trying to be inconspicuous?!