I work with young children and have read numerous studies about brain development. Learning to crawl is an incredibly complex operation for a developing brain, as it has to send signals to various parts of the body as what function to perform at what time. I would much rather see an infant on the floor moving his limbs around and strengthening the overall body. It's not natural, in my opinion, to 'skip' progressive aspects of child development. Crawling is a big achievement for a baby and they too have feelings of pride. If you've ever watched a baby in the learning process, you can see this. Crawling also enables the child to learn how to pull themselves up and stand with support which helps strengthen their body parts including the neck, arms, abs and legs.
I think early tummy time is both beneficial and essential for babies to learn how to crawl. I've never been an advocate for walkers and didn't use them with my children. I've read more against them than for them. The more babies use their developing brains, the more developed their brains will become.