Except the ability to resize your UI widgets. If your UI is almost entirely static, then sure, bundling a buttload of images is fast, easy on your designers, and your engineers won't hate you.
But if your UI is of the sort that has highly variable sizing and layouts, then you probably do want to start writing custom draw code that removes you from having to maintain a hundred variants of the same asset in your bundle - say, multi-line gradient buttons. It also allows easier abstraction (say, tinted gradient buttons) without your designer having to painstakingly generate what is essentially the same asset save one feature.
Programmatic UIs also make it easier to do related A/B tests - otherwise testing a button's color would involve shipping updates with every candidate in the bundle.
Like all things, be judicious and smart - though the trend I'm noticing is that we're moving beyond simple UIs on iOS, and there's increasing demand for the sort of UIs that demand this level of flexibility. The performance of recent iOS devices have also been such huge leaps that the penalty of not using pre-baked images everywhere is minimal in most cases.
Custom drawing also has a lot of optimization use - this is a surprising little-known trick for custom UITableViewCells. Compositing is still a very heavy load, avoid having complex view hierarchies for high-performance UI components (a UITableViewCell is high perf in most instances). Consider flattening your views into something that fits into your drawRect call - drawing text directly instead of UILabels, drawing images directly instead of UIImageViews, etc.
Also, a surprisingly little known trick is CALayer.shouldRasterize - setting this flag will rasterize everything in that layer and below and cache it. This incurs a heavier hit on redraw, but for objects that do not need to be redrawn often it's worth it - this allows you to have baked-image performance while still doing custom drawing.
But if your UI is of the sort that has highly variable sizing and layouts, then you probably do want to start writing custom draw code that removes you from having to maintain a hundred variants of the same asset in your bundle - say, multi-line gradient buttons. It also allows easier abstraction (say, tinted gradient buttons) without your designer having to painstakingly generate what is essentially the same asset save one feature.
Programmatic UIs also make it easier to do related A/B tests - otherwise testing a button's color would involve shipping updates with every candidate in the bundle.
Like all things, be judicious and smart - though the trend I'm noticing is that we're moving beyond simple UIs on iOS, and there's increasing demand for the sort of UIs that demand this level of flexibility. The performance of recent iOS devices have also been such huge leaps that the penalty of not using pre-baked images everywhere is minimal in most cases.
Custom drawing also has a lot of optimization use - this is a surprising little-known trick for custom UITableViewCells. Compositing is still a very heavy load, avoid having complex view hierarchies for high-performance UI components (a UITableViewCell is high perf in most instances). Consider flattening your views into something that fits into your drawRect call - drawing text directly instead of UILabels, drawing images directly instead of UIImageViews, etc.
Also, a surprisingly little known trick is CALayer.shouldRasterize - setting this flag will rasterize everything in that layer and below and cache it. This incurs a heavier hit on redraw, but for objects that do not need to be redrawn often it's worth it - this allows you to have baked-image performance while still doing custom drawing.