Well, the issue they're having is with their current pipeline. In the simplest case, it's:
Base Delay -> Delay Reduction (Dual Wield, Martial Arts, Delay-%, etc.) -> Adjusted Delay
1) Adjusted delay -> TP formulas -> TP/hit
2) Adjusted delay -> Various forms of Haste -> Effective melee delay
What they would need to do instead is:
1) Base Delay -> Delay*(1-MIN(1-0.2/(1-Haste%),Delay Reduction available) )-> TP formulas -> TP/hit
2) Base Delay -> Delay Reduction -> Various forms of Haste -> Effective melee delay
So they'd have the monkey around fairly deep in their own machinery and add a new function or two, exposing total Haste earlier, and changing the pipeline. Specifically, they'd probably leave effective melee delay calculation the same and add a new function to feed in to the TP Formulas. It's not impossible, but I can't really fault them too much for being afraid to touch that code.
Edit:
For the record, such a change would amount to a ~5% DPS increase for Dancer when I implement it in the spreadsheets. I made the assumption that equipment Haste would still be capped during WS.
I assume it would be similar for other DW/Martial Arts jobs (probably a slightly larger DPS boost for some), so it would have given Haste Samba value in this way. Not spectacular, but a buff to Haste Samba nonetheless.