Right, but none of that has any effect on your own tweet stream (unless you follow spammers).
If you are talking about searching, the only way search is valuable is if a service gains critical mass, and the only way that happens is if the barrier to entry is low enough that enough users join the service, which will include spammers.
Spammy @mentions is what blocking is for. Also, you don't get @mention notifications if you don't follow them (see settings. may or may not be enabled by default).
You can only DM people where you both follow each other.
I get almost a dozen spam mentions or mentions directed at a misspelled username everyday. That adds a lot of overhead to my usage that get old to the point where I don't even want to read my mentions steam anymore. Certainly less frequently than I used to.
I'm @derek on Twitter, so I have the same issue. If App.net ever got popular, these names would have the exact same issue. So, that's not a compelling reason to switch.
I'm assuming that "spam" is largely either @spam or follow-spam. Also, I suspect that blocking @mention after @mention gets a little old after a while.
Actually, I think the quality of your stream will improve. People value things more if they pay for it. In this case, you pay for being a part of a community. It is expected that people will try to keep/improve the quality of the community.
If you are talking about searching, the only way search is valuable is if a service gains critical mass, and the only way that happens is if the barrier to entry is low enough that enough users join the service, which will include spammers.