An obvious point maybe, but it isn't free
Airlines will typically be billed per character sent and received.
Also, I don't believe there is a standard ACARS fit. On the Boeing at least, the 'application' is designed by the airline (etc.) so you might have a button here for getting weather, or a screen there for submitting defects, and perhaps another page for sending a list of the crew members on the flight.
Re the loadsheet - if you're working for an airline which does the final loadsheet before you close up, it's great having sight of it before the dispactcher brings it up (they may well be busy doing bags, pax, cargo, etc.) - you can get on and do the performance (for example) and then just check the figures are correct when the LS arrives.
As far as getting confirmation of the figures after push goes, you work out your performance based on an estimated loadsheet, and then simply check the figures are roughly correct once the doors have shut and you're on your way. If you're out by more than, say, half a tonne (on a large jet), it's not really a massive deal. A tonne will probably change your V speeds by a knot, and in any case, do you really think the DOM of the aircraft is correct to the nearest kg? On a deep maint check, you can easily get a tonne of water out of the insulation cladding!
NW3