In most cases I understand that the primary bottleneck to bootstrapping a node is the TX verification. 

IBD would happen much quicker if validation could be skipped, in favour of instantiating a specified UTXO set. 

If we skip downloading the blockchain data altogether, we would only be able to operate as a pruned node. Pruned nodes lack a lot of functionality, like the ability to robustly support a Lightning node, running on top. 

If assumeUTXO lets you get a functional node up and running, while the IBD takes place in the background, that's a good step in the right direction.

If the provision of a UTXO set could in some way negate the necessity for validation, as part of the IBD, that would be even better....