>>57558147a) if you ever found out that your tulpa had inconsistencies with the person you based it on then it could cause you distress. Imagine someone making a Mumei tulpa before she said that blueberries are mid, but they made the tulpa like blueberries because of the berry meme. disastrous.
b) what if you stopped liking the person you based the tulpa on? you're just stuck with that, you can't just turn the tulpa off when you get bored of it.