Mana is like money and pretty much everything in this game costs a certain amount of mana to use
It is not as good a threat although it does offer some reach but given that it is also half the price, is playable in many more archetypes and is also a mana ramper and fixer The drawbacks of this card as life gain are that it is slow to get life returns from it, it will situationally not work for life when no dead guys are there to be exiled, it is reasonably easy to kill and it cannot give you a big single injection of life
Meanwhile Rashmi and Ragavan can be played as just "generic goodstuff" which I generally avoid, though it's much stronger and more interesting if you actually run topdeck manipulation and artifacts to take full advantage of their ability
(A permanent with hexproof and indestructible can't be the target of spells or abilities your opponents control
If we get more toxic coming up in March of the Machine and/or other future sets (depending on the result of that set's story) then maybe we can see these be relevant in the future
That it hits for seven instead of six is just gravy, and I definitely dont hate trample