I agree with a lot you have said here, the Grass Types that evolve by Stone are:
Leaf Stone - Gloom to Vileplume, Weepinbell to Victreebell, Exeggcute to Exeggutor, Nuzleaf to Shiftry, Pansage to Simisage
Sun Stone - Gloom to Bellossom, Cottonee to Petilil, Sunkern to Sunflora, Petilil to Lilligant
Shiny Stone - Roselia to Roserade
Water Stone - Lombre to Ludicolo
Looking at the above list the stone used to evolve the most Grass Types is unsurprisingly the Leaf Stone with Five with the Sun Stone very closely behind with four. The Weed Pokemon Gloom evolves using both Stones making it one of few Pokemon to have multiple evolutions depending on Stone used.
It is also notable that the version Counterparts of the various games (Gloom-Weepinbell, Lombre-Nuzleaf and Petilil-Cottonee) all evolve by Stone with it even being the case that the Hoenn Counterparts evolve using contrasting Stones. (Water and Leaf)
As you stated evolutions by Stones is very useful for Grass Types as they become more powerful Pokemon. The reduced number of moves they gain may be to prevent the player getting an early game breaker.
In the Kanto Games you can buy as many Leaf Stones as you like in Celadon City as well as in Gen 6. If say Vileplume was able to learn moves at the same levels as Oddish and Gloom it could give players an early boost. Of course the player could leave the Pokémon as an Oddish until they learn their final move but that would have the trade off of having them in the weaker form for longer.
In some cases the moves Pokemon learn on evolution are completely different to their pre-evolution. An example of this is the Lilligant line which only learns the various dance moves in it's Final Form. This gives the player opportunity to decide how to raise their Grass Pokémon. :)
Whilst levelling up is a logical and straightforward way for a Pokémon to evolve, evolving by Stone gives the player opportunity to evolve their Pokemon as soon as they have the stone or as late as they wish. :)
