Well for instance, maybe you are down to 10 mana in a big Nightmare run, so you want your last cast to be more powerful (though taking longer to recover mana from)

For the -50% one, maybe your chance of success is less on an opponent, so you would rather have 4 chances to cast something on them instead of 2, even with less damage.

Or maybe you have 11 mana and realllly need to MS 2 different people (even if for lower durations) so you rather 1 mana casts instead of a 2 mana cast.

It is for situational flexibility.

For instance, perhaps with the 200% damage, it may cost double the mana/runes but your MV might now reach a higher chance to remove spells, which might be more worth it (with less chance of it reflecting back on you because 1 cast instead of 2).

Lots of neat little situations that would make it interesting to use.

Oh, Pillz, thanks for your constructive criticism, always welcomed and appreciated.