Blackberries indeed work much, much, much better with a data plan -- but I can understand that even a Blackberry without a Blackberry plan (i.e. SMS only) works better than carrying two separate devices: A cellphone and a 2-way pager. There are advantages to a Blackberry without a Blackberry plan over the older cellphones;
The Blackberry WAP browser may not be able to be used without a Blackberry plan unfortunately (To the best of my knowledge). It is possible there was an error that allowed you access to the browser. It's too well-integrated into the Blackberry infrastructure and very few people get a Blackberry without a Blackberry plan.
You need to install a generic 3rd party browser: reqwireless. You will need to install reqwireless and configure the reqwireless-recommended WAP settings, and you should be able to do some basic browsing. The cost will be $30 to purchase reqwireless, but you can download the free trial and try it out;
Blackberry has some extra infrastructure on the cellphone network, compared to regular cellphones, and costs more to run, so having a Blackberry is more expensive.
Also, unless you specifically saw the three letters MDS, T-Mobile was not marketing MDS, but a Blackberry plan. The Blackberry plan *and* MDS are two completely different things. MDS is not necessary for email or WAP browsing per se. Also, MDS is available for only $10 per month from
http://www.myblackberry.us .... but you must already have at least a basic "Blackberry plan" to use it;
To the best of my knowledge, you can get unlimited email and web browsing with some Blackberry carriers for something like about $40-$50 per month, so it should not cost you $1300 per year for unlimited email and unlimited WAP browsing. (I am assuming you are in USA). If you want MDS extensions to the Blackberry plan, to do additional things like instant messaging, you can get MDS from a third party for much cheaper. (remember, a Blackberry plan is different from MDS.)
However, the treatment you got was probably a little bit strange, especially if you have 5 cellphones (Or 7 if including the two-way pagers) --- they should be treating you better and giving you the discount!
Let me emphasize there are three main levels of Blackberry usability:
Basic phone plan -- cellphone, SMS, and standard WAP only (reqwireless)
Blackberry plan -- Adds email and Blackberry-enabled WAP (HTML too with some carriers)
Blackberry plan+MDS plan -- Adds everything else, third party Internet apps including chat software; etc
Usually, it's most economical to get MDS separately from your carrier, as MDS service can be purchased from a third party (just like you can purchase email service from a party different from your ISP).