This is the standard dogmatic view that plagues competitive play. Grass/Flying type... And somehow it's already being written off and we don't even know anything about it except it's type! I find this quite incredible to be frank. And yet you mention Chesnaught, Celebi and Venusaur in the same breath, all with their own double-weaknesses!
Okay just because something has a double weakness doesn't immediately mean we write it off as nonviable. Grass/Flying already has a track record of being well... extremely weak and nearly useless.(Sparring Shaymin-Sky ofc) It's just not a good typing at all offensively and defensively. Compare Tropius to Shaymin-Sky for example:
Tropius Grass/Flying
@ Chlorophyll/Solar Power/Harvest
99/68/83/72/87/51
Mediocre stats at best, two situational abilities, and one that Tropius can't utilize well at all because of his crippling weaknesses. His speed is awful, with below mediocre offensive stats and good bulky stats. You'd think glancing at the stats this Pokemon would be a great bulky Pokemon just based on it's defensive stats. 99 base HP is pretty high, and combined with 83/87 defenses can make a Pokemon pretty dang bulky. However, Tropius dies easily to any ice move and due to it's extremely slow speed, won't be lasting long in the arena against anything with a super effective move. It's got a decent movepool at best, but it can't make good use of really anything due to it's stats and abilities.
Shaymin-Sky Grass/Flying
@Serene Grace
100/103/75/120/75/127
Shaymin-Sky. It's got the ability Serene Grace. It's got strong moves that are boosted by Serene Grace.(Seed Flare with 80% spdef drop, Air Slash with the 60% flinch) It also has outstanding SpAtk and Speed combined with pretty good bulk in 100HP, 75def, and 75spdef. It can outspeed tons of things and get an almost guaranteed Spdef drop on Pokemon that switch in to it, and then you can flinch them to death with Air Slash. You can also run Substitute/Leech Seed/Air Slash/Seed Flare and go bulky speedy. Your SpAtk stat won't really matter as it's high enough and you'll likely be dropping the foe's SpDef stat by two stages with every Seed Flare hit. By doing this you can sub/leech seed and pretty much hax your way to victory. Of course, it still suffers from ice moves, but few things outspeed it.
Tropius suffers from everything about it. Shaymin-Sky suffers from it's typing, yet it also benefits by getting the STAB Air Slash and Seed Flare that it's ability benefits. Nothing about Tropius' typing benefits it at all.
Yet, Shaymin has a BST of 600. Starters usually have around 520, that's a lot to work with, of course, but is it enough to get Rowlet's final evolution to be viable in competitive if it doesn't have a good HA like Gale Wings or Aerilate? It's pure speculation of course, but undoubtedly Grass/Flying is more of a con than it is a pro as a typing for Pokemon in the competitive playing field.
Also, Chesnaught, M-Sceptile, and Celebi are all very good, you know why? They have the movepool, abilities, and stats to back them up. Chesnaught is a great bulky leech seeder that has Spiky Shield as well, but is barely beaten by Breloom, as Breloom has Toxic Heal and Spore. M-Sceptile is extremely fast and can only be paralyzed via effect spore and glare, as it is immune to all electric moves, in fact it gets a SpAtk boost when hit by one. It's got sheer power behind it with a 120 STAB Leaf Storm at 145 base SpAtk. I'm not even sure if a 2x Contrary Serperior beats that. It also has access to Dragon Pulse and Focus Blast. It can also go physical and mixed as it has 110 Attack. Celebi is another straight 100 through and through with stats. This makes it very versatile as it can be bulky or offensive, and it guess what? It has the ability and movepool to boost! It's a very great Baton Passer, slap a weakness policy on it with Nasty Plot and you can easily baton pass a 3x Spatk and 2x attack over to one of your benched Pokemon. It's got natural cure as well, which is useful when you pull off a nasty plot and get statused, then you can simply baton pass out and lose the status.
