If u feel something does all u need then it's only natural taht u stick 2 it... though I am afraid eventually u might need something else quite soon... I know acid 4 can handle virtual isntruments and some midi.. but as far as I know it's isn't vary flexible or easy 2 work this way... Acid 4 is brilliant if u work mainly from loops but the degree of synthesing and transformation is rather limited (unless u use some preatty powerful softsynths, efxs)... still working with this "limitations" can b a challenge 2 produce good/ different music..
IMHO there is so far u can go with just sampling and audio sequencing... unless u have a very vast and RICH collection of samples (and I'm not talking about loops)...

my 2 cents..

Z