I have a Zumo with the XM radio and I also got the weather/traffic service, as it was only 4 bucks more a month over the regular radio subscription. I do however, live close to St. Louis , MO, which is on their Big List of citys. The weather and traffic are very handy to have, but I do not use them that much around here. But I did not buy my FJR to just ride around town, I bought it to do some Touring, and when covering vast distances, having the GPS with the weather and traffic is just too cool a tool to pass up. Really, if your just going to ride around you area, what the hell do you need a GPS for anyway...much less traffic and weather, you can get that on TV before you go out.
But if I planned on riding down to Florida, (which I will this summer) I can get upcoming weather reports from several major cities along the way, and if going through the bigger towns, the traffic info would be great. (My recent ride through Memphis showed me how usefull this type of info could be)
Also, with hours in the saddle, the XM radio helps while-away the time.
The XM "antenna" for the Garmin units is not just a regular xm antenna, but is in fact the recieving unit , the Garmin just acts as a controller, thus this explains why they are expensive. The GXM reciever does work with any of the Garmin GPS devices that have the proper interface, so if you switch from say the 2730 to a Zumo later, you can keep the XM puck and use it on the new unit. Also, as these puppies are expensive, the market for selling a used one is pretty good, you can recoup alot of your investment if you feel the XM thing ain't for you.
The problem of course for you is if you do not want the XM "radio" but just the weather and traffic, you still have to pay for regular XM subscription...not sure if it would be worth it this way, but I'd just get a pair of nice earphones and force myself to rock out.......
KM