The strongest note you can play against a chord is another chord tone. Learning arpeggios helps you realize where the strong notes of the scale lie. It's also great for training your ear to hear notes like 9ths 11ths & 13ths. After awhile you will start to think chord to chord when soloing rather then key, and you can add color to your phrases by altering the 5ths (b5 #5) and 9ths (b9 #9) and 11ths (#11). Learning arpeggios does wonders for your sense of phrasing too.

welcome back to the theory posts Christoph, its been awhile since I seen your name posted here.