you've summed it up yourself - you don't need a telephone/landline, therefore you aren't paying out for BT line rental, possibly just so you can use broadband on it.
Another obvious advantage is the fact you can use it nearly anywhere. I say nearly as this is one of the drawbacks - I use T-mobile broadband via my phone as a modem....it works most of the time but I'd describe it as flakey. For example three days last week I had no access at all as the transmiter was down. Was visiting a friend in Manchester at the weekend and there was no 3G/HSDPA signal at all - now central Manchester is hardly the scottish highlands where you could expect poor service.....so you really need to investigate whether it's going to work, and how well it is - before you jump in.