Taken from namecheap:
Entrepreneurs will be thrilled to discover that Namecheap offers .io domains. The .io extension is the official top-level domain for the Indian Ocean, but it also has a geek feel to it, because I/O refers to input/output. This is a great choice for tech-centric sites and startups. Like all other Namecheap domains, it's backed by our stellar service and support. Per registry regulations, no .io domain may be used, directly or indirectly, for any purpose that is sexual or pornographic or that is against the statutory laws of any nation. In the event that NIC.IO learns that a specific site breaches this condition, the registry reserves the right to immediately deactivate the offending registration. The applicant may seek the reinstatement of any suspended domain name by asking for an arbitrator appointed by the World Intellectual Property organization.
In short, of course you can register them as long as it's memorable to your potential customers, then why not
