The problem is that the feel of a professional grade instrument will be a lot different, so someone who isn't in the early beginning stages won't really learn much about playability. It's like getting behind the wheel of a '67 VW when you're used to driving a Ferrari.
An early learner doesn't know the difference because they've never driven a Ferrari, so the VW is fine for that learner to master the basics, but eventually more advanced players (and drivers) want higher quality - and once you've gotten the feel for that, it's not really useful to go backwards because you feel like the instrument is fighting you when it won't do what you want it to do as easily as you're used to.