No, it's not a requirement, but it does limit what we can do for you
As a HubSpot solutions provider, we're experts in the full application of the software. It takes a good amount of skill and experience to be a HubSpot expert, so we don't expect our team to be virtuosos when it comes to every other software on the market.
Whilst we don't insist that you have HubSpot, this may limit what we can do if you are using other systems. We can still help with aspects such as producing your strategy, creating content, performing SEO tasks, paid media, but there are certain things that we'd ask you to do. For example, landing page builds, sales reporting and email automation.
We'd ask for the opportunity to give you a demo of HubSpot and how powerful it is. You'll undoubtedly be able to see for yourself why we've taken this stance.