My iPad has cellular connectivity. It's an option. If you don't have it on your tablet, too bad, not my problem. iMessage IS an asset for someone like myself.
Everyone in my professional services firm uses iPhones, as do my friends and family. I love being able to text from my iPad or Mac and having it all synced to my phone. I don't want to have my google account, logged in through a web browser, to use Google Talk. Or be tied down to using WiFi on my iPad.
And the concept between having iMessage and being able to blindly send a message to a phone number, is that it auto detects the number and will send it as an iMessage to the iPad and Mac.
On Android, I cannot go to my contacts, send a message to my friend's phone number, and he won't receive it on his phone, tablet or laptop.
But wait! Let me guess, I can finagle around with third party solutions on my Android phone, tablet and my computer for something that fixes this problem?
Again, as the posts and original topic detail, this thread is to point out that for me, a user who has an iPad, Macbook Pro and iPhone, it is much more convenient and seamless to live in Apple's ecosystem than move to another. Maybe you prefer using your web browser and Google Talk, along with adding in another contact field (email address) and using another application on your mobile devices, as that is somehow more convenient for you. If so, great. Not for me.