I never viewed summoning as really a combat thing. Remember Jiraiya told Naruto that it was really more of something to use when you're desperate and that for the most part is the way it has been used. Naruto only used it against Gaara because he was desperate and at the time he didn't have any other high level techniques aka Rasengan. Orochimaru or I should say Kabuto only used summoning because Oro couldn't use most of his techniques because his arms were sealed. Jiraiya also couldn't use his techniques because of what Tsunade did and Tsunade only used her's because the two of them were using their's. Tsunade didn't even use her's much she only used it to preoccupy Manda so she could attack herself.
The only other attack type summonings we saw in the first half of the series were Kidomaru and Tayuya and I trust I don't have to explain those. Sasuke used Manda as a desperation move so he could escape Deidara's explosion. Jiraiya summoning Gamabunta against Pein wouldn't really have worked because I don't think Jiraiya wanted to have a battle on such a large scale and since Pein seemed to be the master of summoning I'm sure he would've had some massive combat type summon he could use in that situation.
The main reasons people don't use summons are because they usually change the scale of the battle and I don't think most ninjas would be able or would want to have a large scale battle. Secondly summoning is supposed to use up a massive amount of chakra and the amount is supposed to be proportionate to the size or number of the summon. Remember when Naruto first summoned Gamabunta he had to use the Kyuubi's chakra just to do it and I'm assuming it used up most of the chakra he was capable of at the time. So again most ninjas won't use high level summons are summoning at all if they have sufficient high level techniques that they could use without having to waste chakra on summoning.
Jiraiya didn't use Gamabunta and again I'm just assuming because he obviously felt that if it just came down to a battle of summons Pein would win. He said that the only way he would be able to beat Pein was if he got him one on one and two because he was conserving his chakra to use hermit mode because using and sustaining it obviously requires a large amount of chakra.
Frankly I'm glad Naruto doesn't use summons it doesn't fit his charge at everyone fighting style whether that style is good or not summoning doesn't fit it and because if he used the summoning that much then I won't be able to give him credit for any of the battles he has had.
I don't think much people use or know summoning techniques because someone would have had to come into contact with the creatures before and then they would have to sign a contract and then the scroll would have to be passed on etc. I always viewed the summoning as something that should be unique to Sarutobi, The Sannin and the people they have taught and for the most part it has been. Sasuke, Suigetsu and the Sound ninja were Oro, Naruto and the fourth were Jiraiya. I don't think Tsunade has thought anyone yet and the only other examples I can remember are Pein and Hanzo.