You should ask your client have they ever experienced down time in the past which their provider could not explain? And if their provider said a server was DDoSed, where they the intended target or was another server or router attacked which affected them indirectly?
I think people are overly worried about getting DDoS with all the hacking incidents in the news. It's rare for most companies to be randomly targeted. If they are a big corporation who have reason to believe they will be targeted, they should discuss that with their hosting provider first before relying on some third party application which i'm sure they are paying for that may not be very helpful in the event of a massive DDoS.
If they've never had any down time they shouldn't need to pay extra. There are plenty of applications for windows and linux to monitor for port scans, DDoS, tutorials on setting up good IPTables/IPChains rules to limit types of packets to ports etc.
IMHO the mitigation service is just playing on companies fears of a future DDoS attack.