I don't have a REx, but especially if you drive where there are fees for ICE use, definitely run the REx on the motorways, and use EV mode in the city. Except in the USA, well, maybe NA, you can turn the REx on any time the SOC is 75% or lower. Keep in mind that high speeds with the heat on, lights, etc. your battery charge could still be dropping while using the REx. On average, it will be able to hold, but extended heavy use will cause it to drop. That's not a huge issue unless your SOC gets really low, then you'll have the equivalent of around a 38Hp (I think the 2017 added a few HP - it is 34Hp on earlier versions) car...just how fast can you go with 34/38Hp? That is plenty, on average, to keep the battery up, and momentary needs for more comes out of the battery, but not when the battery gets low. When low, the computer will limit the max drain on the battery, and that can mean it will shut down some things like climate control, and finally, limit speeds or acceleration, depending on how low you actually let it go.
IMHO, and from BMW docs, the car is designed for commuting and around the city. While, especially with the REx, it can go longer distances, as many have demonstrated, it is not the same as driving an ICE, where you have full performance until you run out of fuel, and you can refill that tank in a few minutes verses maybe hours, depending on the charging equipment available to you. England has a better network of CCS charging stations than most of the USA (well, the square miles it needs to cover is SIGNIFICANTLY smaller), and some areas, like CA in the US have fairly decent coverage, that is mostly the current exception. That is slowly changing (thank the lawsuit against VW for some of that pending infrastructure), but not fast enough for most. Your maximum economy is to run the vehicle in EV mode all of the time. The REx is only good for the low-mid imperial 40MPG or so. Lots of ICE, especially numerous diesels, can best the i3 REx when running in that mode with the batteries mostly discharged like you might have when trying to travel longer distances.
Some of it depends on your tolerance for the realities. When running in EV mode, or when you blend the ICE running in REx mode plus what you get with recharging the batteries, it still is fairly efficient, but no where near as efficient if you can stay in EV mode all of the time. Mine is a BEV, but I don't need to travel that far...when I do, I use my ICE. It's nice to have that choice. Personally, I'd not go that route, but your question was could you, and I think the answer is yes, but at what cost in convenience? Are you willing to have to refill the gas tank daily? If an anticipated charging facility is non-op for the day, will that seriously impact your life and schedule? It will be a long time before charging opportunities are as common as petrol stations, if ever in our lifetimes.
2011 535i x-drive GT, 2014 i3 BEV