It's a good idea to boost intelligence, if you decide to keep your Swordsman around for two skills in particular, these being Dark Seed, and Breakdown. The Dark Seed skill combines with the Shaman skill Hex of Darkness to produce the killer spell Hell Fire. And the Breakdown skill combines with the ranged spell Chaos Wind to produce the killer spell Destructive Wind. These two spells may be a good reason in the long run not only to boost your Swordsman's Intelligence, but also to simply keep the Swordsman around into the highest levels of the game.
But, there's a catch. The Shaman is eventually outclassed in the later levels by the Oracle, a healing class which is capable of healing your entire party, or of casting highly damaging AoE spells. It's highly likely that you'll still have a mercenary with the skill Chaos Wind, however, in the later levels of the game. The Gunner and the Artilleryman both have Chaos Wind in their arsenal, and each is a highly damaging class in their own right, oftentimes being kept around by the most knowledgeable players late into the game.
Another important thing to consider when deciding whether to keep the Swordsman (or Swordsmen) around who have served you so well in the earlier levels, is those Class C tanking mercenaries we talked about earlier in the article - the Lady Knight, Beast Tamer, and Spartan. Each of these, in the long run, are a stronger tanking class than the Swordsman, at the higher levels.
There's almost no reason to keep a Shaman around, once you are able to train an Oracle, so is it worth keeping your swordsman around for the Destructive Wind spell? This could be argued either way, as the Destructive Wind spell is pretty powerful. But most of the top winning Korean builds do not have Swordsmen in them. If the best of the best don't use them in the high levels, then it's probably a good idea not to use them once you have the Class C tanking mercs available to you.
Additionally, both the Viking and Spearman also have the Breakdown skill. It is arguable whether these two classes are worth keeping around into the later levels as well, but for very similar reasons to the argument against keeping the Swordsman. In the later levels, there are simply better mercs to fill those precious party slots with.