Personally, if I played with my rule on the table, I would use the X extra damage along with those powers that still deal half damage on a miss. Also, they seem like a good choice to co-op with your team and drop a dangerous Elite or Solo quicker than normal. My point is that the X extra damage has its fair amount of uses too.
The X extra attack could be also useful to increase your attack especially against Soldiers in almost every attack against them.
I used to use MM1 monsters most of the time but I also included MM2 monsters as well as monsters from other books (MM3 is one of them but I am not 100% sure). Before I stopped playing for real-life reasons, I was mostly making homebrew monsters by relying to DM's Guide 1 and 2 guidelines as well as my own experience. Unfortunately, I came up with this rule at the end of my DM career and not earlier. :(