However, it's not specified to completely disable speculation, only "to the extent that such speculation can be observed through side-channels as a result of control flow speculation or data value speculation". So e.g. an implementation could decide to disable speculative memory access while still allowing arithmetic operations.
Preventing Spectre vulnerabilities is pretty much the only reason you'd want to disable branch prediction though (except for curiosity of course). Without prediction, performance would be as bad as if every branch was mispredicted, since the pipeline has to stop and wait at every single branch. The idea of branch prediction is that the "wasted" cycles waiting for the branch to resolve can instead be used to do some computation that may or may not be useful; if it turned out to be useful you saved some time, if not you didn't lose anything (besides maybe some electricity). So even a branch predictor that randomly guesses with 50% accuracy is a huge performance win over not speculating at all.
