It is, of course, possible that Amantadine will work even if it is taken only 'when needed', but my gut feeling is that you will have to either take it regularly - or not take it.
Requip alone worked well for me for about four years, but I found that if I took more than 8.5 mg per day, I felt worse in many ways. Too much Requip made my tremor worse, affected my feet adversely, and made me faint at times.
I added Amantadine before I added c/l 25/100, and it helped somewhat. I have just now read ol.cs's explanation, in another thread, of the combined effect of c/l and Amantadine, and I can see why I feel well on my regimen of three half pills of c/l 25/100, 8.5 mg Requip and 100 mg Amantadine per day.
I also try to take as little of as few medicines as I can, and the Amantadine makes all the difference in my daily 'cocktail'. It tames my tremor, keeps dystonia away and gives me back my balance.
It does make your head feel 'fuzzy' at times, eyes feel blurry, ears buzz slightly, but with enough rest and enough sleep, this is not a constant side effect.