I imagine there is a weight penalty to this, but I imagine the most prominent argument about this notion is that if the rim comes out of true, it is going to be very difficult to correct it, if at all possible with a cast wheel. Simply because the wheel is 'casted' doesn't make it immortal or impossible to bend. If you hit a harsh pot hole or something while out you may be able to solve the problem on the road side with a spoke wrench when using a spoked wheels, I am sure it'd be much more difficult to fix that sort of problem with a cast. In other words, in a spoke scenario you will probably be able to get home, the severely bent cast wheel probably means you need a pick up.
Another note worth considering is the aesthetics. Personally I aim for a more subtle look, as much as possible, a cast wheel is sure to draw the eye to the hub and give the bike a very unusual look. So if you are hoping not to draw the maximum amount of attention to your electric bicycle, a cast wheel probably isn't the way to go.
I know nothing about your wheel building skills, training and experiences, nor the wheel builds you are dealing with, but there is a lot to say on the subject concerning building the spoked wheel. There are some really excellent books and videos to watch that might help you grasp the concept as deeply as humanly possible. http://www.wheelpro.co.uk/ I like plenty of this guys book. http://www.amazon.com/gp/product/B004H1UA9Y/ref=as_li_tl?ie=UTF8&camp=1789&creative=390957&creativeASIN=B004H1UA9Y&linkCode=as2&tag=xbnijgbr-20&linkId=MV6QABVQOQAK5Z32 http://www.amazon.com/gp/product/B00H49Q4KU/ref=as_li_tl?ie=UTF8&camp=1789&creative=390957&creativeASIN=B00H49Q4KU&linkCode=as2&tag=xbnijgbr-20&linkId=OVMWTKV3L5S2ESTQ I also feel these videos are worth watching.
Setting up the wheel initially correctly does a lot for keeping it true, as high tension as your rim can handle is a wise course of action. Sapim also makes locking nipples that might help. But beyond that, suspension and less weight is surely going to help. When I say less weight, I mean especially the hub motor itself (assuming you are already using one). A lot of people likely use wildly too heavy hubs on their electric bicycles and it is sure to contribute to a wheel going out of true sooner rather than later. Suspension is a bit of a broad subject but an insanely important one in my opinion. Not everyone knows that the higher the air volume the tire is the better it will likely do for absorbing shocks small and large. So if you can fit a wider tire, I'd jump on that for sure. In fact, I'd go so far as to seeking out, having one made or fabricating a new frame yourself just so you can fit wider tires. Probably sounds extreme but cast wheels aren't exactly a casual next step. There are some really nice air shocks out there with excellent adjust-ability that should add lots of softness to your ride.
I atctually considered the idea of trying to figure out how o have a company make a 26" version of a cast wheel for a hub motor, but the aesthetics notion really kills it for me. Naturally, I'd pair all the softness of a wide tired full suspension bike with any electric bike setup, but especially a cast wheel.
