There are so many situations where the Fairy-tail guild gets into really tough fights and in serious trouble!. Gildarts is extremely strong, how is that he is not very involved in fights??
As said before Gildarts is a class S mage and he is almost always adventuring. For example there was an episode when Gildarts came to the Guild and everyone was doing a party.(I don´t remember in which episode it happened thought). Even if he wanted to join the fight´s he would need some "days" to reach the "battlefield".
As others have mentioned, Guildarts isn't always around. You may recall there is a special board with S rank jobs for S rank wizards, and those missions are intense. So while the guild may be having trouble with one thing, Guildarts is likely far away handling a different job of immense difficulty as well. He simply may be busy and unable to leave.
http://fairytail.wikia.com/wiki/Gildarts_Clive The above link is a bibliography about Gildarts Clive and it also mentions why he doesn't fight often, not in a more clear way though. One of the reasons why I think he doesn't fight like he used to be, after the casualties he faced in the battle with Achonologia which nearly killed him. Yeah, but if you want more info just take a look at the link above.
1.He isn't around most of the time, doing so called decade or century quests as he is a S-Class wizard. 2.Natsu and the guild need to show how strong their plot armor and friendship power can be, instead of raw firepower. 3.It's no fun to watch OP characters enter mid fight and defeat villains with ease.