To piggy back on Otaku's statement: I think here is a great example of a stage 2 that provides a reason to be played along with the "how". Note, as Otaku stated above, this type of synergy is EXTREMELY uncommon and as far as I can tell completely unintended by the lords of Pokemon.
Here is a full evolution line stretched across two completely different expansions (Black and White / XY). If you were to play either full evolution line for these cards within only the expansion they came out in, they would do exactly what Otaku said they do. Either the stage 2 is underpowered compared to the amount of time / effort to get it out along with being underwhelming compared to its contemporary EX, or the stage 1 is complete filler and provides absolutely no benefit for you other than setting up your stage 2. Note, even in the example I am presenting, the stage 2 is *not* the primary focus of the deck. It provides benefits without necessarily being required to be out ASAP. About the only line I can think of off the top of my head that provides a useful stage 1 along with a stage 2 focus is the Greninja line. I am sure there are others, but I am not a historian.
Here we go:
Tynamo (pick one, I just happened to pick this one)
Eelektrik (Dynamotor ability to get [L] energy from your discard onto a benched pokemon)
Eelektross (Energy connect ability allows unlimited basic energy transfer from benched pokemon to active pokemon. Electricannon attack [L][L][C][C] hits for 80 with a choice to discard all [L] energy and hit for 50 more)
Based on what I put in the parenthesis, it should be fairly obvious how these synergize. Get three Eelektriks on your bench, level one into an Eelektross, = consistent energy flowing from your bench to your active for pokemon that require that kind of flow (in my case, I use this combo for a Rayquaza / Eels variant in order to have a second attacker for Jolteon EX / EX block abilities + a way to get energy up to Rayquaza without requiring a retreat).
In theory, this setup seems like the type of thing you would absolutely drive toward as fast as possible in order to "keep the energy flowing" so to speak. In practice, it completely depends on the deck and in my experience, you don't actually want the Eelektross until late in the game or until you need an alternate attacker. And even in the decks where I have this setup, Eelektross only makes an appearance about 60-70% of the time and is discard fodder in every other game. And this is an example of an evolution line with TONS of synergy.
Now take a different line. Let's go with the Delphox line. The stage 2 Delphox from Fates Collide is an absolute beat stick once you have it set up. Basically, count all the energy on the board (both yours and your opponents) add 20 for each of them. All for [C][C][C]. That is an amazing attack. Absolutely amazing. The line even has a Break evolution that allows you to dig into your deck and find a fire energy and attach it to a pokemon. Furthermore, if you go back to the earlier XY base set iteration of Delphox, there is a variant that acts like Octillery, but you get 6 cards instead of 5. So why doesn't this line rule the world? Simple. It takes too long to get multiple Delphoxes set up and there is nothing that speeds up the process enough to get you there without severely hindering your ability to do anything else.
Greninja decks *can* work without the use of water duplicates to get multiple copies set up, but you can take it to the bank that the deck wouldn't be a tier 1 deck without that attack. It would suffer the same fate as Delphox because the setup requirements would be far too great. Why? Because with Greninja (as with Delphox) it is understood that you are *going* to lose pokemon. They will go down and go down quickly in this environment. The only way to counter that is to set up enough of them that you have multiple ready to go while getting more back. That's not the only thing that makes Greninja great, but it is one of the primary reasons why it works consistently.
Long story short, Greninja has synergy up and down the line to get itself up and running fast enough to compete in this environment. Delphox has a top heavy synergy without the ability to replenish itself in time to effectively counter the speed of powerful basic / EX decks.
I am sure there are better examples and I didn't do the Greninja section justice by any stretch, but I think this illustrates some reasons why most evolution lines simply don't get played in any competitive environment.
Sorry for the book