To be honest I don't see the point of paying for a phone line/SMS service on my mobile any more, since I can use apps such as Skype and Whatsapp to perform the exact same role, using cellular data only. However most companies refuse to provide unlimited data only, it is always bundled with calls/SMS packages raising up the price. Would it be worthwhile for me to buy a prepaid card for my phone instead, simply using the cellular data for any calls or messaging I have to do?
Well, the only question to that is do you have a phone that will allow you to only use prepaid cards? If you are using a phone that originally bought with regular service, then the switch will most likely involve replacing your phone also and if that's the case, then you should simply switch to a company that does have unlimited data as there are plenty who offer it.
Depends on how much you use it. If you're a big cell phone user, and use a lot of data and minutes I'd suggest just going ahead and signing a contract. It's typically a better deal. If your someone who only uses a cell phone lightly and in emergencies then I'd just go with the prepaid plan. It's more practical for those type of situations.