As lots of Blackberry users have just discovered reliance on an external supplier is not necessarily a good idea.
A RIM Server goes down and takes out the system. Chances of recovering the huge amounts of data whilst maintaining service are not high.
Once a big on-line system is broke it is very difficult if not impossible to fix as you are trying to add new transactions to possibly corrupt data.
Ask the Inland Revenue.
BTW, I will claim copyright on the Complete Loss of Your Data phrase as I I fear it will be in increasingly common usage.