If you just want to speculate on EUR/USD, then 6E is fine and in many/most cases the best option.
But:
The ability to adjust position size exactly to the exposure you want is important when your positions is not just a major cross, but exotic or a basket consisting of more than two currencies.
I'm following close to 50 crosses, and my trading rarely includes Cable and Euro although USD/JPY, USD/CAD, AUD/USD do appear. But I mostly have signals triggered in more exotic crosses like CHF/PLN, EUR/ILS, GBP/CAD, NOK/SEK, AUD/NZD etc. For positions using these crosses there simply are no futures, or the spread when replicating a spot position with several futures is far worse than the interbank market spreads. The liquidity is also far worse.
So, for simple outright strategies futures is fine. But if you're doing more sophisticated stuff you have to trade spot. Period.