$15/mo is still too high for dabblers. It should have a free or cheaper tier (with low quota, or $0.20/hr, bill as-you-use like AWS) for people who want want to fiddle.
IMO they should just use a cookie to let you try it out for a couple of days, and then tell you that you need to buy the product. It's easy to circumvent, but a ton of people would just choose to pay for it.
I also think that they should have some kind of student pricing. Many students couldn't afford $15/month, but they might be able to afford $15/year.
Except Wolfram should just restore Wolfram Alpha to its once free glory (instead of the annoying nagware that it's become) and also offer this Wolfram cloud stuff at a reduced price.
The end goal for them should be to convince people that they need to purchase Mathematica. Pricing at $15/month means they're only really attracting existing Mathematica users who want the convenience of the cloud when they're away from their primary machine.