There are some really, really strong card effects that can only be accomplished with gimmicks. I do what basically amounts to T. Wonder's Wild Card handling. If it didn't have enough gimmicks to it, I put the packet in a Himber style packet switcher at the end, that way I am clean if anyone asks to see the cards...........Personally the way I handle this is say Spec A asks to see the cards. I call them over and take them aside and say, loud enough for everyone to hear, "Ok, but you can't tell anybody how it is done." I then show them the normal cards. People will ask them latter and they will either believe him/her that all the cards were the same or they will bug them the rest of the night trying to get the "secret" out of them.
I also do a repeat card to pocket. The first two times the card goes to the pocket are pure sleight of hand, the third time uses a gimmick--a very, very old gimmick that I have used to fool magicians under the age of 40 quite badly.
One of the best do it yourself gimmicks I ever learned was from a Darwin Ortiz book. Yes, Darwin Ortiz. So even one ofthe most famous mechanics isn't above using them to accomplish his goals.