If you buy a wireless router from your Internet service provider, that router will have been tested for compatibility with your Internet service. If problems occur, your provider will be on the hook for support or replacement. If firmware updates are issued, your provider will test them and may even install that firmware automatically.
However, the wireless router offered by your Internet provider may not meet your needs. because wireless routers supplied with residential broadband are not business-grade devices. They are usually entry-level routers designed for home networks. A small office or business may have additional requirements that would be better satisfied by another device. For example, perhaps you want to protect your business network with a Unified Threat Management (UTM) appliance that provides on-board anti-virus, anti-spam, anti-spyware, intrusion detection, and VPN/firewall capabilities. Or perhaps you need to deliver WLAN coverage with more capacity or range than a single wireless router could deliver.
The bottom line: Don't let the tail wag the dog. Before you even look at your provider's router, decide what your office needs in the way of an Internet firewall or UTM device, and what your office needs in the way of wireless network infrastructure. If those needs don't match up with your provider's router, then buy your own device(s).