I agree with bclemens. Unless you already have a Sirius/XM subscription, the cost is $16.95/month. (current subscribers can add an additional receiver for half price). For something you'll only use periodically, it's an unreasonable expense for most.
Yes, you can cancel the subscription and restart it later (at additional expense), but that's a hassle. Installation is so sinple even a caveman can do it, so for those who want it, purchasing an aftermarket receiver and antenna makes more sense than for everyone else to pay for what only a few may use.