Let me begin to say that I think they're fairly overpowered at lower levels (same as Yugo pots, but that's another thread altogether). I know this is not everyone's opinion, so please stay civil if you disagree and please avoid using /notsigned without any supporting evidence/argument.
The reason: While at higher levels +2 (fully stacking) stats are the chocolate icing on the bacon cake that most toons can do without, at lower levels they are very powerful (perhaps too much so). It's silly a lvl3 Wizzie (with +2 stat item (+4 if you can get bear endurance clicky), +2 Shrine, +40Hp Yugo pot, false life spell) can sport 100+ Hps. Similar with STR, for STR-based toons (20 base, +4 bull's, +2 Shrine, +2 Yugo) 28 STR on a lowbie is pretty disgusting. I used to use PnP as a reference to highlight the big difference and power creep of DDO, however, now it's gotten so far and worse that referring to PnP for comparison is border-line useless, which is the main motivation behind my displeasure with the current stat Shrines (and yes, Yugo pots as well).
My proposal: Make them give stats at the same rate as tomes. Lvls 1-2 cannot be used at all. Lvl3-6 they give only +1 stats. Lvls 7+ the shrines give +2 stats as they're now (for those guilds that have them). While this doesn't address the Yugo pot issue (which I'm sure can be discussed at length), it does address the issue of power creep and number inflation currently pervading DDO.