illiquid, I guess I'm surprised to see someone like you raise this old question, again. Maybe I can help. For me, the choice between currency futures and spot fx really has nothing to do with "if it ain't broke..." It's all about selecting the right tool(s) for the job. Sometimes either one can get the job done; sometimes only one will do. Here're just 3 major cases (there are more) when spot trumps futures hands down.
1. Crosses. Take a look at the liquidiy of any non-major pair on GLOBEX:
http://www.cme.com/trading/dta/hist/daily_settle_prices.html?type=cur
For all practical purposes, the liquidity is basically non-existent. 15 contracts in an entire day for GBP/JPY; 205 for EUR/JPY; doesn't even register for CAD/JPY, with open interest of... 7 (gee, thanks); 38 - 108 for EUR/GBP; and so on. End of story.
Even for all the majors, except maybe the lone Euro FX, depending on the size of the portfolio you're running, currency futures liquidity can be rather poor or inadequate most of the time, outside the first half on the US session, roughly 8:20 - 11:30 am EST.
2. Daily interest differentials, reflected as actual cash debits and credits, not as continuous changes in basis. I run an entire strategy that is designed around that.
3. Exact, optimal position sizing / money management / risk control. For instance, continual compounding: I want to be able to adjust the next trade size after every closed trade, win or loss. That alone makes a world of difference in long-term returns. Can't do that in futures, period.
In fact, here's #4... to which you already referred to with "the ability to trade small lots." That's a biggie, wouldn't you agree? This applies to probably 95%+ of all "small speculators". That includes anyone with an account size of a few thousand dollars... or even $10-15,000... who jumps into the Euro-fx fray (or any other major currency future).
Yep, it's one of my favorite forex subjects on ET: leverage. Given the size of the futures contracts, they're immediately forced into double-digit leverage. Right off the bat. 1 lot. Nothing they can do about it. Thinking they can survive (and prosper on) such leverage and extract profits from something that apparently, these days, every schmo on this planet, fully equipped with a dial tone and a cursor, is swinging at. Right.