Hubspot configuration with potential future merger into Salesforce

I have a client that is looking to start in Hubspot with the eventual goal of using Salesforce. They know Hubspot won't solve all their problems, but it's free and they purchased a few licenses to start out.

They are looking to add their customers as 'free' users to input deals. I am on the fence between my customer (the manufacturer) entering their reps as customers in the CRM, or the alternative of adding every rep (20 or 30) as a free user, and all them to enter their deals into the system. Any drawbacks to this approach? I explained the adding them as CRM users means the SF merger will mean needing a license for each…