When you pay for hosted monitoring (even just a basic uptime/ping solution) you're paying for three principle things:
The pretty reporting interface these services usually offer
(Because you have to show this stuff to the big bosses and justify the cost!)
The uptime/availability of the service provider
(Because what good is it if your monitoring system is down more often than your real site?)
The luxury of having someone else maintain the monitoring software.
If you're satisfied you can get (1) from the tools you want to use, and that a VPS will be adequate in terms of (2) there's no advantage to a hosted monitoring service beyond (3), which has never been enough for me personally to fork over money to someone else. A half-day every few months to vet and upgrade monitoring software isn't too high a price to pay in my personal opinion.
Edit to add: Some of these services do offer valuable additional functionality - @Alex mentioned one such feature: The bigger ones have geographically distributed locations and will tell you if you're unreachable from (e.g.) Thailand, but up available to your users in Chicago.
This sort of functionality becomes more important as your geographic coverage needs expand.