Dekaranger is one of the best Sentai series ever.

Honestly, I think that the cards were a better idea all-around.

Because, seriously, cops should not double as executioners, I don't care who or what's supposed to be in that little box, that is not a system that makes me comfortable.

Grumm... he's one of those villains who is boring, but effective. He comes off as a real threat ala Xanatos always benefiting no matter what happens, but personality-wise... he's just dull, and his motivation is... I'm not even sure what it IS, and I've seen that ending at least twice.

SPD is a decent Power Rangers show.
Dekaranger isn't a very good Sentai show.
So they both kind of suck, in the end.

It's not without precedence! Think of the Terror Toad from Mighty Morphin' Season 1! Granted, the circumstances are a little different as his horn did things where Grumm's were just a symbol of pride, but still!

The motivation might be just the same as above, Grumm's pride! He lost, and he isn't the type to accept such failure!

Also, I looked into things a little more. Apparently, the 10 Years After Special gave a little extra to go on in terms of Dekaranger "Death Penalty", which would still be realistically controversial, but things don't occur in a few seconds as they were made to appear in the show: http://powerrangers.wikia.com/wiki/DekaBase

But then there's the whole... "The Magnficence/Omni" thing... which is still a weird thing I have trouble understanding. Like... what is it? Why does the creepy skull man serve the brain in a jar?

It sticks out in a show I otherwise like as a very weird element that never really sat well with me.

It wasn't always clearly defined, but as stated earlier, it's not without precedence! Apparently, PR looks at ALL details more closely than one would naturally assume: http://powerrangers.wikia.com/wiki/Omni

What is most curious is that Omni has no genuine body, and S.P.D. is set in the future! Either Omni is a type of entity that naturally has no body, or he lost his original one during some off-screen events and needed a new one, which implies that Omni is the type of creature who could easily "come back" and act using his original body! Saban simply has to get the memo...and give the Disney Era its due!