While Selphie may not possess the highest base battle stats in the game, she is a well-balanced character with some unique characteristics that often come in handy. I won't deny it: I have a tendency to put my favorite characters in my constant party regardless of their skills or lack thereof, so I'd always have had her at all times anyway
(also, the nunchaku is, like, sooo cool!); however, I did find her rather useful, unbiasedly speaking. This article will cover my own opinion as an experienced gamer on Selphie's advantages and drawbacks as a fighter.
I should probably start off by pointing out that Final Fantasy VIII's GF/Magic Junction system allows you to turn any character into pretty much whatever you want them to be, regardless of their base, unjunctioned stats: certain items (Str Up, HP Up, etc.) and character abilities (Str Bonus, Mag Bonus, etc.) permit the maximization of most battle stats, while others, like Str +40% or HP +80%, will boost even the more neglected characters' values. With this said, unique abilities, such as Limit Breaks and specific weapon attributes, are what truly should be taken into consideration when choosing who to place in the party, because they can not be modified. When it comes to these factors, Selphie has clear advantages in comparison with other possible party members; for one, her ultimate weapon, Strange Vision, is the only one in the whole game with a 255% Hit% stat (other than all of Squall's, that is, because he's amazing, etc.). In more practical terms, this gives Selphie max accuracy, meaning she will never miss her target (yes, even under the Blind status).
As for Limit Breaks, the matter is far more arguable. As explained in more detail here, Selphie's unique Slot spells can pretty much kill any enemy in the game, bosses included: just use Full-cure on undead monsters and The End on all others and victory is guaranteed, even against Ultimecia or Omega Weapon. Furthermore, this Limit Break grants the player premature access to Mid and High Level magic; together with Full-cure and Wall, this makes for a very reliable battle source in the early stages of the game. It is true that its usefulness may weaken when (and if) the Auto-Protect and Auto-Shell abilities and Megalixirs become available; however, if you're not an obsessive, perfectionist gamer
like me, it will probably come in handy to the very end.
So, all in all, while Full-cure, Wall and magic in general may decrease in importance as you progress in the game, Rapture and The End will always be effective, and pretty much make your party invincible: exactly how is this advantage debatable, you ask? Well, I personally refuse to make any real use of either of the two aforementioned abilities. Like Odin/Gilgamesh's Zantetsuken or Yojimbo's Zanmato (Final Fantasy X), they are just a little too cheap (although, mind you, using them is not cheating, and you don't have to feel guilty for relying on them!), and I'd rather do things the hard way – that is, by making my characters stronger and depending on their skills. Basically, objectively speaking, Slot is probably the deadliest Limit in the game, but I'm more of a Renzokuken kind of girl.
As a final note, and although, once again, a character's base battle stats are not too relevant in Final Fantasy VIII, it can't hurt to know that Selphie's unjunctioned Luck stats are the highest out of all the characters' (Luck being by far the most difficult stat to maximize), and that when it comes to Magic and Spirit she is second only to Rinoa, while her Speed is the best right after Irvine's. On the other hand, her HP, Strength and Vitality are the lowest in the party.
In conclusion, Selphie is well worth using. It may be true that I would have had her in my party even if she sucked, but she definitely doesn't; as one of the most eclectic characters in the game, she is extremely handy.