You need to realize starting strength is predominantly about teaching new lifters how to lift and use a simple, but effective program that can be built up as they advance in strength.
Rippetoe is not opposed to assistance exercises if they serve a real purpose and are effective. The intermediate stage of lifting ie. when you transition from making gains every workout to making gains once a week or so is the best time to start using assistance lifts.
To build a strong, attractive, functional body you do not need alot of assistance exercises, but you do need some or the smaller muscles can actually hinder the big lifts.
At a minimum I would recommend shoulder pressing for a different angle of pressing than just dips, some strict pressdowns to assist in strengthening the triceps in an isolated fashion because they may not be getting maximally fatigued in the compound lifts. Back extensions, leg curls, decline situps, sidebends etc will all help the big lifts.
Assistance exercises shouldn't be shunned. They aren't that taxing either because they should be performed as "assistance" lifts with lighter weight and strict form. Sets of 3x8-12 being ideal. I just did an assistance workout tonight after benching and squatting heavy on monday and tuesday and basically just did decline situps, db sidebends, lying leg raises, back extension and some curls. All for 3 sets of 10. I got some muscle fatigue and blood flow into the muscles and will add a little weight next time. Wasn't particularly taxed from the workout, but rather invigorated and it was beneficial.
There will come a point if you are only doing nothing, but the basic compound lifts that you will get strong enough that you will either injure yourself because the assistance muscles haven't been getting taxed enough or you will stall out because these smaller muscles are becoming weak links. It's just the way it is.
This doesn't mean bookoo assistance. For my bench I'll do close grips or decline dumbells and strict cable pressdowns to make sure the triceps get isolated.
For squats I'll do some high rep leg presses and some abductor/adductor work.
For deads I'll use back extensions, db rows, stiff legged deads etc.
Doesn't have to be alot, but you need to shore up the big muscles. You're only as strong as your weakest link.
P.S. Also, your abs do get a good workout from heavy squats, presses and deads, but they get accustomed to this very fast. They can be made even stronger with a little direct attention and this is really beneficial when you're squatting or deadlifting +400lbs as it will help keep you solid.
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