Actually the statement that a shadow mask provides crisper text is incorrect, at least as far as the broadcast and corperate computer industry goes. Since a shadow mask has more metal surface to it and is generally only anchored to the tube at the corners, as it heats up the metal expands and tends to push out in all directions (it is like a big peice of solid metal whith pin holes punched in it) which distorts not only your text, but the entire picture. An aperature grille uses half the amount of metal on the actual screen surface and is anchored fully on the top and bottom (it is like thin metal threads running vertically), the metal will still expand, but to a lesser degree as most of the expansion occurs on the horizontal plain. One reason for the clearer picture is the electron beam isn't fighting as much metal to actually hit the proper color phosphers on the screen, another is that since the aperature grill isn't expanding as drastically in all directions (compared to a shadow mask) the electron beam has a much easier time in actually hitting it's true designated target on the actual screen. Generally, an Aperature grille Television or monitor will run on less power and produce less heat to acheive the same brightness level of picture. some say that since the gun and overall monitor/television run cooler that it will also last longer. The two retention wires in an aperature grille are there to prevent the thin metal wires running verticaly in the aperature grille from vibrating. Unfortunately if they wern't there, the wires in the aperature grille would start to vibrate as it heated up and that would cause a distortion in the picture. It is the lesser of two evils.
Just look at the broadcast market (i.e. TV stations) and generally you will find that the majority of them are using Trinitron (aperature grille) technology for their monitors. Although some people might say that is because Sony is giving them deals on the product, I can tell you that even with those deals Sony's are still more expensive than the other major players, but yet 60%-80% of the broadcast/industrial market still use the aperature grille technology. I have had experience where a TV station scrapped their entire non aperature grilles after less than two years, due to clarity issues and complaints from the broadcast technitions who have to stare at these things all day long.
A quick and easy test to try is take two similar quality monitors, one with an aperature grille and one with a Shadow Mask (say a Sony vs. NEC scenerio) and place them side by side. When both monitors are both first turned turned on in the morning (cold start), display a picture with complex text in the centre, sides and corners. Come back to those same monitors, after they have been running for 8 hours and display the same image. Although both will probably still have an excellent picture in the centre, the sides and corners on the shadow mask will most likely be bowed or distorted to a larger degree than the Aperature grille monitor.