I don't know what to say, except that I disagree. I think his dub is perfect for the character.
the truth is is that the Roger Smith voice just doesn't fit Spike Spiegel. It just doesn't.
It's realistic. Not everyone shows a huge amount of emotion when they speak. What you described sounds melodramatic.
On the other hand, Spike is a totally different character. He can be the cool and confident bounty hunter one minute, the goofy victim of a prank the next minute, a pissed-off guy the next, and a sad and mournful guy after that. Spike obviously has a wide range of emotions, a range that doesn't fit with the Roger Smith voice. The Japanese voice actor has a huge range, while the American one doesn't. Spike is really too calm in the dub, isn't goofy enough when he needs to be, and is overall pretty flat.
Meh, everyone has different opinions.
When I grow up, I wanna be a 13 year old girl hacker collecting bounties. -TylerL