The one reason I would think of for not doing the upgrade would be if Dish is trying to lock you into a new two-year contract, if you are not sure that you will actually want to stay with Dish that much longer. Is Dish offering you the Hopper 3 for no upfront cost? Or are they offering a different model of Hopper for free, with the option to pick a Hopper 3 instead for an additional fee?
One other reason for possibly not doing the upgrade would be the increase in the DVR fee. With your ViP612, I assume that you are still grandfathered at the old $7 per month rate. Any regular Hopper model (1, 2, or 3) would have a regular-priced $15 per month DVR fee. (I am not sure if Dish is offering you an introductory reduced rate of $10 per month or not. Even if they are, the rate will still increase to $15 when that offer expires.) On the other hand, the Hopper Duo has a regular-priced DVR fee of only $10 per month.
Any Hopper (even the Hopper Duo) would give you more recording capabilities than you currently have, plus the ability to access internet-streaming channels directly through your satellite receiver. If you would like to stick with the limit of two tuners that you currently have, still gain access to the internet-streaming channels, and eliminate the monthly DVR fee completely, then a Wally plus an external hard drive could also be a good option for you.
If you would like to purchase a Hopper Duo or Wally so that you can do the upgrade yourself and avoid the two-year contract, then I have them for sale in the Classifieds section on this site, cheap! I also have a purchased ViP612 available if you would prefer to stick with what you are used to using, and you ever need a replacement receiver.