It's really going to depend on your own needs as there is no right or wrong answer.
Here is what I mean by a situation that might have you buying or not buying a license. One particular user as a good example:
The Net Liquid cash in this account is $623,000 in US Dollars. I'm going to assume that this might be more than the average person reading this post.
This particular trader I know for a fact is using NinjaTrader free version and has no need or requirement to upgrade. One of the reasons why this user decided to continue without purchasing a lifetime license is because NinjaTrader doesn't have a mobile app.
How to Use NinjaTrader 8 - Complete Beginner's Tutorial / Guide for Traders
You would need to use the brokers mobile app in order to monitor, change and execute any orders. Since this particular trader is very mobile, it doesn't suit his needs even if he executes orders through NT8.
This might also be a factor for you when considering the Ninjatrader free or paid versions but for this user it was a deal breaker. I think this is a good example because if you have an account of this size, clearly the money for a paid license is not an issue.
Any questions regarding this feel free to ask but hopefully we've given you a non-objective way of deciding if you need the NinjaTrader free or paid version.
Regardless, if you're just starting out, getting a license key right away is not going to give you any advantage over others. Building up your account first is always a good idea.