I would suggest against taping only. That was and still is the trend with the xB1's and works for those. It is not a good idea with the second gen's as there is less surface area in contact with the hatch, the leverage ratio is higher on the second gen (increased even more with speed) and being in a climate conducive to hot weather tape will simply not be strong enough alone.
In some ways you're attempting to attach a diving board to a wall be putting tape on the very end of it. This is a vast exaggeration but bear with me... The xB2 only has attachment points on the rear hatch for the spoiler. The spoiler footprint, in no way goes over or around the roof to allow for leverage diffusion (think rock climber holding a ledge vs rock climber trying to be spiderman) which means there is only downforce on a part that only has a side connection. Said downforce is only emphasized by wind, closing the rear hatch and of course, gravity. If it were the tiny eBay lip spoiler it would not be a real concern but given the design, it is simply not suitable for tape only. The leverage ratio has been discussed but also simply the design having the open center with the footprints being on only the outsides of the spoiler mean there will be forces not only pushing down, be while moving they will push in the opposite direction as you are traveling.
Long story short, for your safety and those traveling behind you, please bolt it up. It might be fine for a stretch of time but tape alone with the factory/factory style spoiler is more of a risk than I would take and one I would try and sway others from as well.