A lot of people hate the fact that the PS3 has mandatory installs for some games. This is all rubish, and I'll tell you why!
Modern disc drives are not fast. Not at all. This is why laoding times for many games on consoles tend to be so outrageous. PC's solve this problem by installing the game on the hard drive. It allows the game assets to be pulled much, much faster. Hence, reduced loading times.
A blu-ray disk holds alot of data, 50GB when dual layered (correct me if I'm wrong here, I might be). That is a HUGE AMOUNT of data. The catch is that the PS3's blue-ray drive is 2x speed, and that is not that fast at all. So, sony has two options. Release another PS3 with a faster drive, and a higher price, or they can do what PC's have been doing for years and years. Install data on the HDD.
Games like MGS4 have alot of assets. Alot of data must be loaded and this loading takes time. Precious time. So, MGS4 installs to the HDD at the beginning of every ACT, taking about 3 minutes or so each time. This is not a problem at all.While playing the game, the loading screens between huge areas is literaly under one second. This keeps the gameplay from feeling segmented and allows it to load up HUGE amounts of data throughout the game without equally huge load times.
So, I say this: get used to it, because installing games is the future for consoles. Disk drives will never be faster than harddrives, and in gaming, every second matters.