pkfire isnt really that easy to SDI. its actually quite difficult, and if youre stuck in the meat of the attack, youre like guarenteed not to get out before ness gets to you. even if you SDI it, it forces you to roll or retreat, and it STILL most likely got some damage on you. thats a lot more reward than most projectiles in the game. also, if you took falcos laser away, he would be garbage. if you took peachs turnips, pits arrows, or shieks needles away, it would drastically kill any high/top tier potential they have. ness isnt the only character near-reliant on a projectile for a good neutral game. I do agree that ness' main issues now are in the neutral game (only slightly above average speed, mediocre range overall), but pk fire does make up in the range department for a good spacing move, and he has pretty decent pressure on shield considering magnet DJCs and pk fire.
additionally, despite ness' most likely mid tier status, the difference between top tier and mid tier in PM is like the difference between top tier and ice climbers in melee. a smart player has all the tools he need to take down foxes and falcos.