1.57% is a lot compared to the ease of just leaving a couple grand in a high-yield savings account. Maybe it's just me but it feels wrong to take out a loan against my portfolio everytime I have to replace my MacBook.
that's not an emergency! And having a few thousand dollars of liquid cash in a bank account is no brainer.
A real emergency might be like a roof collapse, where you need 10-20k to fix. Or a medical emergency, or a family issue where a fair chunk of change is needed. Using margin is a good way, and is much lower interest than a credit card, provided you have the witherall to pay it back.