The primary advantage of an applicator is that hay can be baled several percentage points higher in moisture and still keep. In the case of a hardcore baler in alfalfa, I see 24% bandied about by the salemen when using the proper chemicals, versus 17-18% when using nothing. This means you can probably start baling a day earlier (depending on conditions) and you'll also lose less leaf.
In the case of haylage, different chemicals are used so the silaging process is more complete. This results in less wastage.
Does your applicator also have a control box with an in-chamber moisture tester? If see many of the newer applicators using the moisture testers on the fly to determine how much chemical to apply. This should save you money, especially if the hay dries down to 17 or 18% while you're in the field.
Rod