Is MewTwo that good?
IMO it's just a slightly bulkier Alakazam without FS and with HB
Answers
It's the best attacker in the game, though it's not a game changer; if you've got Alakazam and can dodge (not always a player issue as some phones are worse than others), then you don't need the Mewtwo. Heck, for the most part you might not even need a Mewtwo except to complete the Pokedex.
Also, it's possible that Mewtwo might be even less important once Gen3 hits as several of the Gen3 have stupidly high attack skills that make Mewtwo look like nothing more than a dex entry.
There's only 3 Pokemon with higher attack stat than Mewtwo, and only Deoxys will fill the role of a psychic type attacker.
Mewtwo may become worse as a generalist with the release of Slaking and Rayquaza, but will still be the best psychic attacker and psychic counter because Deoxys has even less survivability than Alakazam.
You and I have different views of the word Slightly.
When comparing Bulk Multiple HP * Defense
Alk: 21340
Mew: 35125
That's not slight, Mew is 64% bulkier than Alk.
The attack boost of almost 10% isn't that slight either. For me slight is 5 or less.
Or to say it a different ways: 2 Mew2 in your line up will out perform 3 Alks.
Or to extend the Analogy what you could accomplish in a Raid with 6 Mewtwos. You'd need almost 11 Alkahams to accomplish
Mewtwo has two good fast moves, depending on your tastes, and three good charge move variations
(1) FB - excellent gym clearer against Normal defenders without being a glass cannon
(2) SB - excellent against other Psychic and Ghost and almost everything else, without being a glass cannon
(3) P - well P sucks, but hopefully they will fix this, for the all Psychic attacker that is not a glass cannon