It does make sense for now, but the goal of the wireless industry is to "do away" with landline altogether (hence, most of the plans include free Long Distance).
I say it makes sense for now because the industry needs to make improvements in order for that to become a reality. I hear of quite a few people that when cell sites go down (9/11 for instance ~ tons of sites were out of service either from the blast or all of the traffic) they are very upset because they didn't have landline service. On the flip side, on a day like 9/11, the cell phone saved lives... Also, as you said, many use their landline for internet access, (even though, mine is through my cable). And some handsets can be used as a modem, but it eats up your minutes.
If your cell bill is astronomical, you may want to evaluate a few things... Is it the price of your plan? Are you getting enough "day or anytime" minutes to suit your needs? Does your company charge you for using your phone outside of your local calling area (some companies call this "roaming")? Many companies are coming out with verrrry attractive plans for the holidays that you could probably get in on with low activation fees.
