This is not entirely true, High Arcana is quite good, but it's use has to be considered.
- Spell Power 1 and 2 for 4 levels makes up for "losing" levels of Wizard since you gain a DC on your spells as well as being harder to dispel with Mords. Spell Power 3 is good for some builds that reach CL34 or 37 based on other PRC's since it will put them up to 35/38. And of course every CL past 30 is a 5% better chance for your wards to resist the dispel portion of Mords.
- Mastery of Shaping is fantastic for an Armored Blaster Sorc as you can stand in the middle of a horde (as can your party) and unleash nuclear destruction at will without harming yourself or your allies.
- Mastery of Elements is again, better for Sorcs as they are better blasters. It would of course be THE reason to take Archmage if it worked with Frost Mage piercing cold ability and would probably make Sorc/AM/FM/ASoC one of the most powerful classes on the server...I suppose it's a good thing that Mastery of Elements doesn't work with Piercing Cold...
- Spell Like abilities can be quite nice. You lose a 9th level slot to be able to cast a 9th level spell as an SLA twice per day (or 7th or 8th level slot), losing a slot in 4-6 gives you 3/day and 1-3 gives you 4/day. Who doesn't want to be able to cast more spells?????
Which of the High Arcana's are best is very dependent on your build. DC Wizards which include any combination of Blood Magus, Shadow Adept or Red Wizard can certainly benefit from Spell Power and as shown Sorcerer types can make solid use of Mastery of Elements and Shaping.
Is it worth it? Well, no, to me Archmage is a class you take for the RP... Wizard 30 will likely have better DC's thanks to more feats and less requirements. W/SA/BM is better than it is mixing in Archmage, though taking AM to 4 or 6 wouldn't hurt, the feat requirements would be very tough, if not impossible to manage.
In PnP Archmage is 5 levels with a High Arcana every level, but that would be quite OP on this server, thus the change to 10 levels and every other. In any case, if it is a question of building for power, in most cases you leave AM out of your build. If you are including it for the RP, well, then I suppose you make due with what you get out of the class.
How much is 1 DC worth? It depends... If your talking max DC of 34 or 35, likely not much as most end game mobs that matter will save. However, going from 40 to 41 might be a big deal in some cases, being the difference between 75% success rate vs. 80%, for example.