Hate to break it to you, but affably evil is rare, and even affably evil is still evil. Evil folk, devils especially, are not well-meaning by any measure even when they act like it, and it would just mean Viserys having to sleep with even more of an eye open than he already does.
I thought the biggest source of salt in this thread was Viserys not getting to enjoy any of the good things in life because of stuff cropping up? Diplomancing devils into becoming long-term threats to our plans, position and life instead of just killing them for loot would rather exacerbate that issue, don't you think? Because you can't really think they would ever become anything besides spies, duplicitous 'allies' or 'servants' or what-have-you, of which there are plenty already.
Regardless, this is all rather strange to be complaining about, considering that Viserys is fully qualified to draw up a summoning circle and call a devil to make the proverbial deal whenever he wants.