There’s no harm in it, if you can find more than one that meets your requirements for live trading, later.
You need to do that research and make that decision [I][U]before[/U][/I] opening a demo account, really.
The “key concept” is that you need to do your demo trading in the same place in which you’re later going to do your real-money trading, so that you [B][U]don’t[/U][/B] have to acclimatise to a new broker’s interface, margin requirements, terms and condition, policies and procedures, technical support and helpdesk at the same time as being under the psychological pressures of trading with real funds for the first time. Quite important, this!
One important point when opening demo is to find one that doesn’t expiry in time if you stop using it and brokers don’t charge anything to restore it from the archive. I knew when cases where something like that and traders could simply lost their valuable trading tests, history and experiments.
Some brokers have an “inactivity” charge as well, so keep an eye out for that. Nothing worse than finding out your £1000 is down to nothing because the broker has been taking £50 every month you don’t trade
No properly regulated broker does that (or is allowed to). If that’s an issue for someone, they need to take a little more care selecting their “broker” and appreciate how important a factor where/how/by whom they’re regulated is.
Frankly speaking, I don’t think at all that hard to find a good broker, if you are active trade or part of Forex Forums community, it’s very easy to know which ones are good and which ones are not.