You might have heard of that skill, and how it is the only active taunt we have in our entire Swordsman tree, not only that but it is also AoE. This makes Peltasta the best class to take for those looking to cover without any problems the role of a "tank" in PvE (as per defined on a previous section), making you a nice addition to any party to clear dungeons or other group content. While it is possible to "tank" without Peltasta, picking it certainly makes things easier.
Now, even though Swash Buckling is the most shiny tool of this class, it is not the only benefit you will acquire.
The ability to manually block physical attacks with a shield (as well as passively) become invaluable as it is able to block most attacks from bosses and it is extremely useful to negate the attacks of other classes in PvP.
Peltasta also has the benefit of receiving extra defense and block when using a shield, as well as increasing evasion and health through buffs. Additionally, it also some has a couple decent CC skills.
Unfortunately, all of these benefits are acquired at Circle 1, which leaves almost nothing left for Circle 2 and Circle 3, as they offer very lackluster skills in comparison.
The only reason to pick anything more than Circle 1 of Peltasta is to get a higher level of Swash Buckling that, while helpful, isn't necessary at all.
Previously I covered a section with Earth Tower, and how some builds might provide additional tools to help clear it. One of them is Level 10~15 Swash Buckling, but it is possible to clear this content with level 5.
In my opinion it makes no sense to weaken your build to every other aspect of the game just to be slightly more useful at one particular instance at high level, which will eventually be tuned down or become less of a challenge as more content and levels are added to the game.