No, it isn't necessary.
You're an indie developer. Do what you like. You don't need to write that much script if you don't want to, and who's going to make you? Not your customers, that's for sure. You said it yourself: You've seen loads of successful games that don't require them (so clearly you don't need a screenplay to succeed), and unsuccessful games (so clearly having a screenplay isn't critical to success).
Do you need dialog? Up to you! Not necessarily. Heard of Minecraft? Tetris? Do you think Team Fortress 2 - a game whose only dialog consists of occasional outbursts by the classes - would have a screenplay? (of course its comics would; forget those)
Do you need a script? If you have dialog it would be handy to keep it written down so you don't forget it, yes.
Does your script need to be written in screenplay format? Write it in whatever format works for you - it doesn't necessarily need to be formal screenplay format. Triple-A movies have a stigma around being written like movies, with gameplay only there to shepherd you between cut scenes which tell the actual story. Half-Life, on the other hand, has no cut scenes, with its story told purely during the gameplay. A screenplay format might not have worked for them. A lot of independent games don't do it the way triple-A games do either: this is just my opinion, but I think that's because the independent developers understand how to tell a story in a game and the triple-A studios, on the other hand, do not.
Does it need to be 100 pages? ... No, of course not. Super Meat Boy and Braid have almost no dialog at all. Magicka and Eufloria have moderate amounts of dialog. Bastion and Cave Story have a lot. Take your pick! Do what suits you and your game.