Except you don't send IEEE floats over FIX. The precision is typically bounded. Also, that's more for placing orders.. NASDAQ for instance uses ITCH which is a binary stream.

You're right that you'd be leaving performance on the floor if you used the new function in the hot loop, but it would still be quite handy for recorded FIX messages for compliance, auditing, risk tools, maybe for hedging, and maybe for execution tasks outside the hot loop -- that's a huge surface area.

As for exchanges, CME _only_ had FIX for latency-sensitive order entry until the iLink 3 migration began last year. CME is huge and there are others like it, even further behind. There is plenty of liquidity you're missing if you ignore non-binary protocols.