This may of already been mentioned before...
The use of Kayako and SupportSuite for most users is within an ISP environement. With this in mind, kayako is one of the most effective and useful applications around. The problem is, there is no decent or remotely decent billing applications of a similar level to kayako. ModernBill is simply pants, and useless for those that wish to actually have flexibility. I am yet to see if V5 will be any better.
I really think that from an all round point of view - supportsuite should include its own billing system which has the customisation and flexibility of the rest of the SupportSuite application. This alone would really make
SS a great all round product which in fairness would really outperform the competition within billing services.
Its something I know alot of people would like to see in near future releases.