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Ur-Priest is not as unbalancing as it at first appears. Look at it this way:
Character Level 6, Ur-Priest 1 with 1st level spells, everyone else has 3rd;
Character Level 7, Ur-Priest 2 with 2nd level spells, everyone else has 4th;
Character Level 8, Ur-Priest 3 with 3rd level spells, everyone else has 4th;
Character Level 9, Ur-Priest 4 with 4th level spells, everyone else has 5th;
Character Level 10, Ur-Priest 5 with 5th level spells, everyone else has 5th,
congratulations, half way through your adventuring career you've finally caught up with everyone else.
Character Level 11, Ur-Priest 6 with 6th level spells, everyone else has 6th;
Character Level 12, Ur-Priest 7 with 7th level spells, everyone else has 6th,
congratulations, you've finally pulled ahead of everyone else on highest level spells, at a level that most characters never even reach in the fist place.
Character Level 13, Ur-Priest 8 with 8th level spells, everyone else has 7th;
Character Level 14, Ur-Priest 9 with 9th level spells, everyone else has 7th;
Character Level 15, Ur-Priest 10 with 9th level spells, everyone else has 8th;
Character Level 16, Ur-Priest 10 with 9th level spells, everyone else has 8th;
Character Level 17, Ur-Priest 10 with 9th level spells, everyone else has 9th,
they've caught back up, you were ahead by a total of five character levels, 1/4 of your adventuring career, for the price of being behind everyone else for the fist half of your adventuring career.
You'd be better off going Wizard 10/ Ur-Priest 2/ Mystic Theurge 8 anyway.