There are only a few agencies that use T as an agency logo and put it on all modes of transit. The only ones I can think of are Boston and Minneapolis. Translink puts T everywhere but does not use T as its logo. (Also, Scandinavia doesn't count, because there, T just stands for T-banen/T-bane, just like german U for U-bahn.)
And as for M, you'll pretty much never see M on a bus. Only on the metro network. M means metro, i.e. subway, not just any transit. If you see an M in Paris, or Shanghai, or Moscow, or Tokyo, that is always a metro station. Never a bus, or streetcar, or anything else.
And yes, sometimes the M is the agency's logo. But, that's typically only the case when the agency operates only the subway and not any of the other modes, so effectively the M still just means "metro/subway". For example, Shanghai Metro's logo is M, and so is Tokyo Metro's logo, but neither of those operate buses and thus M is only found on subways.
The only exception for M that I can think of is LA Metro, which puts M on everything and uses it as a logo.
The problem with the Metrolinx T is that it conveys no information - nothing about modes (unlike, say, a bus icon, or an M) and nothing about fares (unlike the TTC logo). Not even anything about the agency operating the service (even though that isn't really useful information)! It just takes up space.