I have no problem renting the modem
Over the years, I have had the following ISP's in this order
Time Warner Cable
In all the cases, I rented my cable modem from them. I know the price has not always been the same, and I know that over the course of time, I probably spent enough money to own my own.
However, if you look at the ISP's that I listed, they changed a few times as I moved. Sure, the @home to Comcast was just a change in name, but when I moved from Illinois to Wisconsin, I had to switch from Comcast to Time Warner. Then, I moved back to Illinois and switched from Time Warner back to Comcast.
Instead of taking the chance that my purchased modem would work, it was just easier for me to rent.
Not to mention that if anything goes wrong with it, its less work to swap it out for another, as its not mine.
A really good example of this is what I went through a few months back. Prior to May of 2012, I had only HSI and Cable TV from Comcast, and was renting a Scientific Atlanta modem, Model # DPC2100R2. Since I was renting, I went down to the local Comcast Office, gave them my old modem, and they gave me a Thomson Internet Voice Modem, Model # DHG536.
A few weeks go by, and I find out that the batteries in it are bad. So, I go back down to the local office with the modem, explain the problem, and in no time, I'm on my way back home with a new modem. This time its an Arris, Model: TM402P.
If I instead had purchased my own modem prior to me adding Comcast Digital Voice, I would have had to try to find a place that sells an eMTA modem that Comcast would support when I made the switch. Not to mention what the heck do I do with my old old modem? Then, when I had the problem with the batteries, I would have had to try to either contact the place I purchased it, or the manufacturer directly to solve the problem, or return it and purchase a new one. Depending on where it was purchased, that could be a hassle (may have to ship it back which could cost money, drive back to the store, etc). Worst case is I would have had to purchase a 3rd modem, which would mean I then had 2 modems that I would have no idea what to do with.
I don't know about you, but that seems like a lot of extra work. In my case, it was so much easier to drive less then 2 miles to my local office, tell them the issue, and swap it out in no time flat.
Finally, if for whatever reason the modem I have is no longer supported on their network, they will swap it out for one that is. I don't have to try to keep up on standards, what works best, what does not work, and so on. Not to mention firmware updates to the device. Its on them to keep track of all that. I don't have to try to figure out if the new firmware is supported or not, and if I don't do that research, and it turns out that the firmware is not supported, then I have a dead cable modem, and may have to go through a lot of steps to get things working (roll back firmware, or worse yet, BUY YET ANOTHER modem). None of these are things I have to worry about, which makes my life a whole lot easier.
Again, people may have a different view here, but I have no problem paying $7 a month for my modem, knowing that if anything goes wrong, needs to be fixed, upgraded or replaced, its on the ISP to do so.