In the agreement you agree to their terms. If you violate or are even suspected of violating their terms, then you could be subject to account closure-- which can be very upsetting if you have not moved funds from your paypal to some other more conventional account.
This link has some good and relevant information.
Or just Google "paypal closed my account".
Paypal is fine, I have one and am happy with it. I just am intelligent about using it, and advise you to be the same. If I know I have money coming in, I keep an eye out for the e-mail they will invariably send me. Then I claim my funds shortly after the e-mail arrives. It is not so bad to have your account closed with $0 in it, so just make sure your paypal has $0 in it as much as possible...
If I had more payments coming through, I might attempt to claim my funds once or twice a day regardless of emails.
As a side note, there are other services worth looking at similar to Paypal and for businesses Paypal is not as good really as a real merchant account which often provides more consideration in regards to the business relationship and may allow more opportunity for you to contest unfair decisions against you.