You send a message:
iMessage: Message goes to their Mac, iPhone, iPod Touch, iPad, MacBook. - iMessage is tied to both AppleID and Phone Numbers. If the person doens't have an iPhone, iMessage knows because it checks the Number against Apple's database when you put it in, and it will send it as a text instead (easy to see this happening, especially on an iTouch).
WLM: Message goes only to where WLM is logged in. If they have it logged into their Desktop, but have no WLM client on their PC, you run the risk of the phone prioritizing the IM service and sending the message to a location where the person won't even recieve it (i.e. their PC is logged in, but they're at the mall). The only thing that may save you is if they're logged into Facebook, since the chances of an Android/iOS user having Facebook/Facebook Messenger installed is very high compared to WLM. The whole "will send to the best place" simply doesn't work with the WP7 client unless it picks FB where it may be decently reliable for the reason above, because the phone cannot predict how people are using their devices. iMessage doesn't need to do this, cause it's on all apple devices, anyways. Any other device, it can reliable send a text message instead (and will).
That's a huge difference in usability/experience.
In that respect, iMessage routing is more reliable, although WP7's Messaging client is a bit more flexible in that it supports WLM/FB and SMS/MMS instead of just iMessage and MMS.
The only thing that iMessage does that is similar to BBM is allow you to chat (like any IM client before BBM) and tell you if the message was delivered or read (which apps like Kik and Cnectd did before iMessage). Other than that it works different. BBM isn't even integrated into the stock SMS client on BB. It's a separate application.
All iMessage does is allow iPhone users to forgo SMS/MMS and use data instead, without having to launch yet another client. And BBM is BB only with no decent Desktop integration akin to what Apple is doing in their desktop OS with iMessage.
Apple doesn't need to copy to steal RIM customers. RIM seems to be pushing them to go
P.S. Android is actually eating into BB marketshare a lot more than Apple is.