That's pure perspective. I play normal speed games myself on Civ IV all the time. Every unit, I find, has its day, even the "useful-for-4-turns" Cuirassier.
If your cities don't have the hammer production to make the units quick enough to utilize them, that is another problem in and of itself. Or, if you make the units and then don't use them, then the problem is you aren't at war and have nobody to use them on, or are an imbecile.
Or we can discuss the thematics of units going obsolete quickly.
When you discover a new type of unit which replaces the old like, for example, you now have the Cannon where before you only had the Trebuchet, it is true that there is now a better and stronger alternative to the Trebuchet. However, in Civ IV as in reality, the Trebuchets will not automatically become upgraded, and for awhile they are still somewhat useful. You can start building cannons, but the obsolete Trebuchets still have one last hurrah. You can still use obsolete units for awhile and they can still be useful for awhile. When the enemy discovers the new tech, they may not have the will or capability to upgrade their own units, or it'll be a slow process. Same with you.
Long post short, in any situation where the technology is advancing fast enough to sufficiently render units obsolete as quickly as they are made, then the military is obviously not your priority.
In any case I fail to see the problem.
