MTE mainly exists to catch the initial memory corruption in the first place rather than to protect specific targets from memory corruption. The current limitation of only having 16 possible tag values makes the fallback to probabilistic protection fall weaker than it could be but it's still very useful and multiple invalid memory accesses are often required. An invalid read is protected against as much as an invalid write. ARM acknowledged the issue of side channels able to leak side channels in certain circumstances and that's being addressed for newer hardware. Bear in mind side channels can be used to directly leak sensitive data too and it's a huge class of issues not specific to memory tagging.