There is “no downside” to having a WiFi+Cellular model iPad - whether or not you have any intention of using Cellular connectivity. In fact, there are functional benefits… and many users choose these models in preference to WiFi Only models of iPad.
Only WiFi+Cellular models of iPad have GNSS/GPS positioning capabilities - this being a function of the iPad Cellular chipset. WiFi Only models of iPad have no GPS capabilities with which iPad Location Services can derive location. This fundamental difference may significantly impact your ability to use mapping or location-based Apps - in particular if real-time positioning (e.g., a moving map) is required.
WiFi Only models can only “infer” their static location from a database lookup of neighbouring WiFi networks for which the geographic location is known, or the geographic location of your public IP address. For static applications in urban locations, where the location of IP public IP addresses and WiFi networks are known to databases, an “inferred” location can be relatively accurate. However, in rural locations, or when using a Cellular/Satellite/VPN connection, location accuracy of an inferred location [if it works at all] may be very inaccurate.
In summary, no downsides - but you benefit from additional functionality and the flexibility of using a Cellular internet connection should future need arise.