Additionally, miners wouldn't know when to increase the block size as the safe method of deploying the block size limit as a hard fork is no longer there. It could result in either nothing happening as miners want to play it safe, or the blockchain forking in multiple ways as miners test out different block sizes. Either way could be catastrophic.
BU will let the user select a given Core or XT BIP (this is still be worked on (BUIP002, probably not supposed to link it here)), so for example if they turned on the BIP101 option, their node would mimic an XT node as far as following BIP101, including the 75% threshold and specific starting block.
Just like today, where if XT were winning Core miners might switch to XT, and if not they wouldn't, it's the same dynamic: if XT were winning, the BU miners would likely set their blocksize settings to BIP101. They can do this even faster than Core miners can switch to XT since it's just a GUI setting, not a new client to download.
Lastly, why would it be a good idea for users (especially non-technical users) to decide what their block size limit is? Not everyone is smart enough to know all of the implications of why a certain block size limit should be accepted or not.
They can just follow Core. BU can be set up to default to Core behavior (it doesn't now, but it's an experimental release; anyone could fork it that way, trivially). I mean, you could say the same about XT: dumb users might try using XT. Could happen. This certainly isn't a security risk, or else Bitcoin is doomed because there's no way to stop people from releasing forks. Yeah I know XT has the 75% failsafe, so then imagine the reverse: everyone is using XT and someone dumb downloaded Core with its 1MB cap and tried to mine but kept not being able to build any blocks because their client rejected all the XT blocks.
Point is, the situation today is that miners and nodes need to pay attention to developments today. They can't just blindly trust whatever Core puts out - and if that's the expectation then we already have bigger problems.