I intend on getting an automag as my primary semi gun in the near future, and because the exact reason Tiny said. My cousin brought this point up to me the other day as well..
there's going to be moment where you wish you could have shot faster/more. In most moments playing pump feels like a challenge in the name of entertainment, but in some moments it will truly feel crippling. Like in a walk-on game, a guy coming up to bunker you on your opposite side? Highly likely to miss your shot when you have to turn around, pump, and fire an accurate shot all in reaction to his move. If his gun shoots faster than yours, he has dozens more chances to shoot you out. You have one ball, and if you're lucky you can pump off a couple more.
That's why autotrigger exists, right? Semi when you need it, but instead of pulling the trigger you pump it.
But I think it'd be cool to see 2 evenly matched players who are experienced stock class pumpers to play against each other. one with a pump gun of their choice and the other with a semi. A primarily pump player with a semi is still going to pick their shots and be nimble and play intelligently. They just have the option to shoot faster by eliminating the pumping movement. Someone who wasn't taught to use a fast gun as a crutch simply won't!
Just my thoughts