I have shot quite a few boattailed bullets, but don't have Marlin Bolt Gun. Where I have seen a difference with boattails is at ranges greater than 250 or 300 yards. Within the 250 yard mark or so, they offer no real advantage over a flat based bullet.
Now, I will add the caveat, that some certain boattailed bullets are inherently more accurate than some flat based bullets, regardless of range. I think that it is more a function of bullet quality than any generic application of the statement I made in the first paragraph. The Nosler Ballistic Tips (while I don't personally care for them on game) are extremely accurate bullets in any rifle I have used them in. The Sierra boattails are also pretty darned accurate bullets. The H'dy boattails I've used are capable of good accuracy, but not so much when compared to the Sierra, and even less so than when compared to the Nosler.
I have no first hand experience with the H'dy SST's. Hornady's Secant Ogive design may have as much to do with it as the boattail. Each rifle is a critter unto itself. What shoots the best in mine may be a flop in yours, or vice-versa. If the H'dy SST's you've used didn't perform to the standards you were hoping for, you might consider a different boattail (Nosler, Sierra, Speer, Nosler-Winchester Combined Tech., etc.) before a blanket condemnation of all boattails in general. If you are happy with the performance of the H'dy Spire Point, then bang away and have fun. If you're up for some experimentation, try a different brand, bang away and have more fun.