First of all, I don't think Eminem should diss Canibus. That's the past, simple.
Secondly, I do think Eminem needs to understand the environment he's in, he really needs to assert himself as the artist he was previously, and I'm not speaking terms of quality because that's impossible, but at least in terms of character, and I don't even mean by making fun of pop culture all the time or talking about drugs or his mother constantly, I mean in terms of the feared emcee he was before that could annihilate you with a few bars. The real hip hop world recognizes Eminem doesn't have that aura about him anymore, whereas unfortunately the hip-pop world still does but they're nobodies anyway, I refer to all of Young Money, the Rihanna generation of hip hop fans and such.
Quite clearly, Eminem has listened to the teachings of Jay-Z and Dr. Dre here, two men who have avoided real beef for over 10 years. They don't want to go down that lane, because they're too worried about what others would reveal and say, and they think far too highly on themself. They don't address issues, they just push it under the carpet and in the general view can get away with it because of their power over mainstream hip hop. But make no mistakes, their legacy is getting more and more damaged in the hip hop world, and when it's all said and done, that's the world that will determine their legacy when they're done, because the mainstream will have moved on to the next Joe Schmo.
I'm not saying Eminem should adopt the 50 Cent mentality either, where he bombs on anyone who even mentions his name, no, that's no Eminem style, only a few men can do that and get away with it, and 50 is one simply because of his character. This is something only 50 Cent can do in the post-Tupac era.
What Eminem needs to do is adopt the Nas mentality in hip hop. Nas RARELY ever gets dissed, and it's not just about respect, it's about fear too. Nas will retaliate, he will go at you, he won't mention your name but the hip hop world will know when its about you. I say this because I think Eminem doesn't want the general perception of him to be a rapper involved in beefs (just as Hov and Dre do). There is no perception about Nas being involved in beefs, he's too smart to get into that.
This is the best example I can give you on how Nas goes about his business. From his most recent solo album, the opening track. To the naked eye, it's just a great song, but to the hip hop world, this is clearly a blatant attack on 50 Cent. This is Nas' cleverness. And it's not even real beef, because Nas silently loves 50 Cent but this is the competitive spirit of hip hop in full effect.
http://www.youtube.com/watch?v=2rOnjXBwUa4
N1ggas is still hating
Talking that Nas done fell off with rhyming
He'd rather floss with diamonds
They pray "please God let him spit that Uzi in the army linen
That shorty doo-wop rolling oo-wop in the park reclining"
Take 27 emcee's put them in a line and they're out of alignment
my assignment since he said retirement
hiding behind 8 Mile and The Chronic
Gets rich but dies rhyming
This is high science
Now add 23 more from Queens to B-more
I've over their heads
Like a bulimic on a seesaw
Now that's 50 porch monkeys ate up at the same time
He does the same with his verse on 'Friends' from Distant Relatives which is further blatant shots at 50 Cent and jabs at Jay-Z on his second verse. Em should take note.