Because someone who carries in an IDPA-approved manner for competition but stuffs a J-frame in his pocket every other day will not have the draw stroke from practice transferred to his normal habits?
That is certainly true. However, while hoping not to step on anyone's toes with this comment, generally a draw from a pocket isn't going to be something accomplished quickly -- not quickly in the "1 second or less" sense that a practiced shooter could draw from a belt holster in.
IMHO, it's just such a different thing I don't see any damage done by practicing a good draw from a holster when taking part in practical competition/practice. If you set up your "speed pocket" rig and are working on shaving hundredths off your pocket draw, then I guess practicing a belt-holster draw might conflict with the skill you're trying to hone in your pocket.
No in competition, you aren't practicing your pocket-carry retrieval but if you want the benefits gained by shooting IDPA scenarios and matches, you're going to be drawing from a belt holster anyway, period. If someone feels that you should NOT shoot practical competition because you might lose your pocket-draw edge if you have to work with a holster ...
... I'd say their priorities are grossly misplaced!
(Or, as seems to be the case sometimes, they're fishing for a plausible sounding excuse for why they don't shoot competition.)
Of course you should be practicing drawing from whatever carry method you choose (on your own time, in a safe manner) anyway. Even if that's pocket, ankle, shoulder holster, or cross-draw.
As Jim suggests, IDPA (or even USPSA!) is a great excuse/driver for many of us to get out to the range 4-5 times a month to sharpen our skills. If you don't do competition, well that's fine, but are you driving yourself to practice with what you DO carry? Do you have the motivation to get out there on your own and push yourself hard against the clock to be a better shooter?
If not, then any excuse about the wrong kind of holster is just so much lying to yourself.