Octopus: I reasoned the same way like you about subs for quite some time (ie - I never bought them). However, subs are the most cost-effective navy-killing unit out there (provided they bring a CV for air cover). Plus they are good fodders…
Now, the only theatre of operation where you regularly actually NEED a navy-killing units is the Pac…
And in many games the Pac sees no or very little action. In those games you never see subs…
Then OCCASSIONALY I have seen Germany purchase 1 or 2 subs in the Baltic, around G3 or G4. In these cases Germany also have bought an AC in Baltic on G1, and now wishes to protect that Baltic navy from extinction from a potential allied attack…
The UK or Russia should never - and I mean NEVER - buy subs…
UK should be able to sink any potential German fleet in the Baltic w/o subs. If it is NOT able to sink a German fleet in the Baltic, it can only be due to the German spending so much IPC on fleet builds, that Germany will lose the game to the Russian anyway. :-)