For Shackled City, which is a pretty tough campaign, having all of your bases covered is important. A lot of tough enemies with really wicked tactics lie ahead of your group and having arcane spells added to your party's repetoire in addition to what you already have will be a huge help. That being said, it's also important to play the kind of character that you want to make. If you want to play a druid, but have your doubts about party balance, why not start out as a druid and take a good deal of levels in sorcerer? You can have druid flavor and arcane spells and maybe qualify for a prestige class that allows you to brush up on both. I say sorcerer just because it seems to fit with druid better than wizard, and if you're very careful about choosing spells (taking only the really essential "nuts and bolts" arcane spells for sorcerer levels) you can use your druid spell levels to keep things fresh and adaptable. Good luck and I hope you guys have fun!
It's true, play the character you want to play. If you really feel obligated to have arcane magic skills, it might be good just to take one multiclass level in an arcane class or max out cross-class Use Magic Device solely for scroll use. If you're willing to get some things scribed and spend some party dough on "the essentials", it shouldn't be as big a problem. Even better, ask your friend playing the rogue to put some points into Use Magic Device so that you won't have to even worry about it. Or even if the cleric player takes Wee Jas as his patron and takes the magic domain, that can help. Not ways to replace a devoted arcane spellcaster, but suggestions to have it both ways some of the time. Sorry to bogart your post like this, I'll leave the floor open for everybody else's suggestions.
Although we are only about a third or fourth of the way through the adventure path, I wouldn't say that a full arcane caster is essential if you had both a full cleric and full druid in the party. There would be the occasional gaps in spells, like identify, knock, invisibility, etc. but if the rogue really buffed up his Use Magic Device or multiclassed into wizard or sorcerer, that might suffice.
Have you considered the Arcane Heirophant from the Races of the Wild Handbook?
Essentially it is a Arcane Caster/Druid who receives bonuses to both casting abilites and also levels gained in the prestige class add to the Druid's existing wildshape skill. Yes you do suffer gaining the few levels or so of Mage or Sorc but its not a total loss of the ability.
First off, most people consither this to be a DM only board. If you're going to play this campaign and want to keep the game fun and spoiler-free, you'd best stay away from these boards from now on.
The Shackled city adventure path is an urban-heavy campaign. It works best when the majority of the party has an attachment to cauldron (prefferably) and it's environs. For the cleric en the rogue, this shouldn't be too much of a problem. If you're going to play a druid (or a ranger or a barbarian for that matter), you should really think about the reason your character is in cauldron so often. Perhaps your DM can help you there. There are a few organisations PC's can join, if your DM will let you that is.
IMHO a true arcane caster, preferably a wizard due to the faster spell progression, is a necessity. I have been running SCAP for 18 months now and my party has no true arcane caster as all three are multiclass. They are all 11th/12th level and have access to only 5th level arcane spells. It is a huge disadvantage, and will only get greater as the enemies become tougher and tougher.
But I will take one for the team if a primary arcane caster is required. Is one nessercary to complete the adventure? I've tried using the search but their are to many spoilers I have to close my eyes to!
In my opinion, a primary arcane caster is not necessary, but would be very useful. A cleric, as always, is a necessity.
If you're really concerned about an arcane caster and it's allowed by your DM, have you considered the Leadership feat? True, you can't take the feat until you reach 6th level, but the real usefulness and need for arcane spellcasting tends to come at higher levels. Just be aware that having an animal companion will affect your leadership score.
Have a great time with the SCAP!