To put it in perspective, they are really fodder. Although they are great for the beginning of the game, giving you a close to end game character for, what I find is cheap. But those spotpass characters fall short quickly, for example, Chrom, your main character, with a full support character (For argument sake, let's say lucina who has 10% attack/10% defend and support stat increase), your chrome will have up to 9+ stats on certain attributes depending on Lucina's class. (For example, a lord adds stats to Speed/Luk, while a General add's 5 str and 7 defense). In addition to that, every 10 stat's increases the support stat by 1 point.
So although most classes cap at the highest is mid 50's in some stats, those support bonus' plus skill's which can even increase that, could end up with 70-80+ caps. Which is essentially adding up to 30 damage, 30 less damage, or a 30% increase in proc rate (which can be as high, in my experience, 80% frekin %. While your spotpass character is stuck at it's caps.
However there advantage is, they are pretty much fodder. Sure you lose the weapons/levels you gained with them, but if it's between them and a story character, you're going to throw them under the bus. They are also great for short term use, (depending on the mode, I mean normal and hard you could probably use them throughout the whole story with minimal effort).