The thing is in Melee there are about 7-8 viable characters where as in PM there's probably 20+, so sitting in mid tier for Marth isn't necessarily a bad thing. In 3.02 he had some match ups that were very difficult to consistently win making him a somewhat risky tournament character (Marth is known for being very inconsistent in tournament due to the mental strain of playing him well consistently). In 3.5 the majority of the cast was nerfed enough inadvertently making him better. He still is going to be a more inconsistent choice than some characters but I believe he's completely tournament viable without absolutely needing a secondary. Some match ups you might want to consider a secondary for depending on your comfort level with them are Yoshi, Link, Sheik, Mewtwo, Puff, Charizard, Ivysaur, Samus, Dedede, Meta Knight, Pit, ROB, and Sonic. These characters have traits that rival Marth's strengths making them slightly more difficult, but they are completely beatable.