Is there a specific reason as to why they want to use that? Are you doing something with level adjustments or anything of the sort?
There is no more Level Adjustments. CR is not LA.
There is guidance in the bestiary on how to adjudicate monsters as PCs. Use those guidelines.
Also, it won't give the PC a corner on the power curve through 20 levels. It's actually not that significant by comparison.
I would not allow any templates to be used, as they are not intended to be applied to PCs.
Advanced can be VERY powerful for a loss of one level. It is essentially +2 to every roll, +4 to several defenses, and many other bonuses. That is VERY powerful for some builds (almost all martial characters), perhaps not so for others (full casters).
+2 to hit and +3 damage (best case), +2 HP per level, and +2 AC is nice. But, past level 10 quickly has diminishing returns. Only the HP aspect of it keeps giving returns level for level.
Similarly, +2 spell DC is also nice, but the extra spells you get don't scale with level. It only takes a 28 to get an extra 9th level spell each day, for example. That's readily attainable by mid levels.
Well it would be +4 to AC, or +6 if you were a monk.
+2 to initiative.
+2 to all saves (+4 for a paladin)
+2 to all skills
+2/+4 To CMB/CMD
Better then a level of fighter, rogue, and probably monk.
Debatable for paladin, barbarian, ranger.
Worse for bard, cleric, druid, wizard, sorcerer.
Well, sure. Two classes really capitalize on the other increases. They also don't get that additional level until a little bit later. However, CR 1 monsters still gain 20 levels. It's just a different progression.