Another thing to keep in mind, security-wise, is that not only can this magus enter the Aegis at will, but he can also grant tokens to others.
In the end, it depends on the overal tone of your saga. How much conflict is there in the saga, that involves other covenants? How much trust is there towards magi of other covenants? Is it quite similar to the question of longevity ritual specialists (do you trust them enough to be in their lab for a season, and certainly be able to gather long-lasting arcane connections). Except that this is a commnunal decision of the covenant.
In any case, some covenants will never use those services, because they are simply too paranoid to do so.
Also, as Jonathan.Link mentioned, there is the matter of Penetration. By RAW, wards and Aegis must have enough Penetration to beat the creature's Might to prevent it from entering. That means that your ReVi specialist must have a Casting Total that doubles the spell level to get full benefits from the Aegis. For a level 60 Aegis, that means having a casting total of 120! That means a Focus as well as scores above 30 in both Rego and Vim.
Note that the Penetration Mastery doesn't help much there, because it basically adds 1 to your Penetration Ability score for each time it is taken. But what arcane connection will you be able to use to multiply? So it ends up only adding 1 to your Penetration Total.
The best way to increase penetration would be through Wizard's Communion, but again there you have some limits. If you want to keep the vis-saving feature of Mercurian Magic, then by RAW all of the participants in WC must be Mercurian as well. And the total levels of WC must double the level of the spell to be cast. And those WC must be variant that have Sun duration, which reduce their effective level by 10. In short, you'd need a cabal of fairly high-power Mercurian magi working together to achieve a full-powered level 60 Aegis. Which means splitting the vis income amongst them.
Not saying it can't be done. Just that, unless you have house rules in place for some of the previous factors, it is not really that easy to get vis from this on a regular basis.