the most alluring thing about simipour by far is its speed, and i think you guys underplay that a bit. ludicolo and samurott are indeed dominant and some of the best pokemon we have available to us, but they're outsped by a load of other very good threats, like braviary, gardevoir, sawk, rotom-a, sawsbuck, etc. simipour struggles with many of the same problems that the special variants of the other two have, including being unable to break through regice and friends reliably. simipour should really only be used if its speed is a necessity or you'd prefer to not stack weaknesses to stuff like sawsbuck (which IMO is one of the best pokemon right now). it's also the only one of the trio with a special boosting move outside of rain dance, which actually lends it quite a bit since it can do a buttload to regice (+2 252 SpA Life Orb Simipour Hydro Pump vs. 248 HP / 8 SpD Regice: 226-266 (62.25 - 73.27%) -- guaranteed 2HKO after Stealth Rock).
mixed simipour is uhhh possible, but really you've got access to a much stronger stab in hydro pump, great coverage in ice beam / grass knot, and even a better boosting move (nasty plot > hone claws). the only thing i would really consider along those lines is acrobatics flying gem for a ludicolo / roselia lure if you're really bothered by them. simipour isn't really outclassed but it's just not as useful as the other two; samurott provides a lot of stability and power in one package and can run coverage moves to bypass stuff, while ludicolo is a sweeping dynamo and can run over a load of stuff in a single turn of set-up, while also preventing stuff like gorebyss from running over a team.