Of course it's always possible to add more and more things to every faction. That's also what we did in the beginning of edain: We just added tons of new units and heroes for every faction.
This lead to ~10 different units in the normal gondor barracks and 3 swordmen/axemen units which all costed between 200 and 400.
It was of course funny to build them all, but new fans were a bit confused as they never new when to recruit which unit and also they were just thunderstruck by the amount of buttons when they opened the menu of the barracks.
Even long time players didn't knew when to build what and games became really frustrating sometimes, as you always thought: "Perhaps I would have won if I'ld have build the other unit"
Additionally this lead to very boring summons, as the summons were mostly normal recruitable units, just with a timer.
Then we've changed our mentality about that and started to make the game more clear for fans by putting some units/heroes in summons or give them some special requirements.
This lead to more interesting spells, as they gave you new units, you wouldn't have without them and a more fluent gameplay. The people had less different types of units to start with (therefore they knew exactly what to build) and with the time they got more and more units. So the players had time to "learn" about the units and it was easier for us to give different units different strengthes and make them more unique.
Just as an example from 3.81:
Imagine if you had been able to build the elk riders normally and you also had the possibility to summon them. The summon would be rather boring, wouldn't it? But as they weren't normally buildable, the summon was much more interesting and many players liked it.
Additionally it is easier to balance all those things when some are in summons. You can restrict some strong units and also add more different designs. Just look at the men of the east. It would be really tricky to balance them with all the different orcs being available. Probably there would always be one superior strategy and either the orcs or the men of the east would be useless. By adding them as a spell, you don't have this rivalry and even in multiplayer games you can have both and it's also viable to have both of them.
Now what does this all have to do with Tauriel?
Tauriel is not only a movie character (there are good movie characters like Lurtz^^), but she also has to have a role in the faction.
As a new hero she'ld need a new spot in the faction which isn't already covered by the other heroes, which is already difficult.
Additionally the more different heroes there are in the game, the less likely you'll see all of them in a game, as you have to spend more money on them. (which is also one of the reasons why we did not make 9 singular nazgul - how often would you really get all 9? Only after 50+ minutes in most games)
So by moving some of them into spells, you'll have the possibility to see all of them in a normal game and you don't have to decide: "Which one is better now? Legolas, Thranduil or Tauriel?", but you can just summon one of them and decide between Legolas and Thranduil (who have completely different strengthes, therefore it's more easy to decide for the player)
Additionally I wouldn't say that Tauriel has a "less important role" as a summon.
You don't always get an outpost in every game! But if you fight you'll always have the spells and can always get Tauriel. Therefore being in a summon gives her some advantages over Legolas and Thranduil.
Finally I personally think that a summon fights better to her. In the movies she was more or less the counterpart to Thranduil in the mirkwood sub-plot.
Thranduil just wanted to stay within his boarders without moving around too much. Tauriel wanted to save the whole world (I think everyone remembers the dialogue between Legolas and Tauriel).
So would it fit for her to be recruitable in the mirkwood main building? I don't think so.
Also would it fit to be always side by side with Thranduil after everything we saw in the movies? I don't think so neither.^^
Therefore I think integrating her as a summon solves those problems really good:
She still clearly has a connection to mirkwood, but you can also see that she doesn't only care about mirkwood, but also the rest of the world (therefore she's not only available when you have a mirkwood-building, but can also fight before that) and she doesn't have such a strong connection to Thranduil, as you can get her in a completely different way.
I think this shows her personality and her role in the movies in a really good way.
Finally one last thing about summons I always like you to remember:
Some summons would be really boring as normal heroes, which is why I like hero summons.
I always take the Balrog as an example:
I played a few mods with the Balrog as a normal recruitable hero. In the beginning it was really funny, but after some time the Balrog lost his attraction. If you compare him to other heroes, then he isn't as interesting anymore:
One ability which makes him stronger (like Aragorn's swordmaster), one ability to jump to another location, one aoe-damage ability and one ability to kill single units.
If you just look at this without him being the Balrog it sounds like an average hero.
But when you summon him, all of his abilities can be a bit stronger and suddenly all of them are really nice.
You care about him much more and you do much more with him, because he is
not always available, but just for a small period of time.
Therefore it becomes much more exciting to play with him.
The same counts for other heroes - even if a hero is boring to play as a normal hero, he gets much more fascinating when you can only summon him and you're always looking forward to get the ability back.
Umfrage