You're right, but for 37€ a month I would never consider a data plan (with or without hardware), moreso if it's capped at 1GB.
There's a competing offer (from another telco) for 29€/month with a 5GB soft cap. The difference over the life of the contract is 190€, not enough to buy a netbook now but probably enough in a few months and I can leave the 29€/month offer any time (if I don't need it anymore or if it doesn't meet my requirements).
Now, your offer for 25€ is much more interesting, but nothing similar is available over here.

Edit: oh, I now found that if I want 5GB with another carrier (e.g. the one offering the netbook), that's 45€ a month, so the difference in 2 years is 480€ and I can almost buy 2 netbooks at that price.