If all you want is the BB messenger, why not get a cheaper phone and use unlimited SMS ($5/phone)? Ultimately the BB Messenger can only converse with other BB users. SMS is (mostly) universal. The whole point of a BB (aside from maybe being a 'status symbol') is its data capabilities. I have mine so that I do NOT have to get (pricey) the unlimited family plan (I use most of the minutes). By using the BB's data plan (+$10/phone) and UMA (VoIP for +$10/phone more) I do not incur minute usage on my bill when I am within wireless access point reach (like at home and work). Secondly, the email only add-on is not all that much cheaper than the data add-on.
I also have mine because of it's usefulness WITHOUT having to fire up a computer or laptop. I can store and view data I need to give out to callers (I help run a youth baseball league) without waiting 5 minutes for my laptop to fireup and can do so conveniently (no carrying around a heavy extra bag to games). Otherwise my Samsung t629 (slider) was handy enough for SMS and phone calls (and even basic browsing with t-zones (+$10/phone)).
It is your money (and contract extension), but if you are going to get a nice phone I would think it would be for the features that make that phone unique and valuable.