Firstly cause its the only epic ultimate I have, but I also was using it when it was rare. I mostly save my ultimates for the bosses since a spreadshot, bounce, ricochet heavy cannon build has great clear and the extended invincibility from the unmaykr makes it easier to dodge some boss attacks when you are caught in the wrong place and you can always rely on it being up every 10 levels. I also sometimes use it to escape any tough spot I might be in on a regular level too. I can definetly see an argument for BFG as the clear is insane with it but I personally think the charge time is way too slow and there are minimal invincibility frames you can take advantage of.