1) platforms like thinkorswim get you 90% there (and have an associated brokerage so you can also run it). You can write powerful trading scripts using a wide variety of signals, but for the last 10% you will need something more powerful like C or matlab or excel.
2) terms of service are always shaky -- if you have alpha, you want to guard that like a first born child.
3) other people (http://tradingphysics.com/) offer market data at very low prices, which is far better than trusting a third party with your code
4) oftentimes strategies don't directly translate to production profits, and to a great extent they depend on simulator assumptions (let's say that there are 10K shares offered at the best selling price what happens when you try to buy all of them? What happens if you are stopped due to RegNMS or some other oddity? What happens if the quote is fake or will be canceled by the time your order reaches the exchange -- a common tactic of Chicago firms like GETCO?) for which you have no control. But now, given that they don't have a BD license, you need to go through someone else (adding another layer and process that potentially could cause problems later on -- I've seen a similar situation happen where a person leaves one company to join another, only to find out his trading strategies don't work in the new place )
Tl;dr: there are better platforms for getting your feet wet, and if they aren't good enough you are better off going with a real solution.