The Merinita (Nature) mystery virtue wilding gives a mage vis:
After spending a season getting the vis, you must spend it immeadiately, but on what?
Aegis, maybe, but other than that?
Healing (after 3 months???)?
The mystery skill nature lore allows finding free vis - without spending a season on it.
So am I overlooking something or is this rather useless?
You're right that the only real use for it is ritual spells (there might be others, but this is the primary use). I wouldn't, however, call that useless by any means.
For one, it generates rather a lot of vis. For example a Nature Lore of 6 produces 12 pawns from a single season. Unless you have a very vis rich saga that's nothing to be sniffed at.
Secondly, while the Nature Lore ability allows you to locate vis in the area, that's limited by what vis is actually present - if there's nothing to find you get nothing, and even if you find some you need to work out how to harvest it, and once you do it's used up until whenever it can be harvested again. Wilding on the other hand extracts fresh vis, so it's not limited in the same way.
Third, the vis can be of any form you choose, so long as a suitable vessel can be found. Searching for vis will only turn up whatever you find - it may not be the kind you want or need. Extracting vis the normal hermetic way only produces vim vis. This makes wilding very versatile for vis generation.
In terms of what to use it on, there is so much more to ritual magic than the Aegis and healing magic. Use wilding to generate terram vis and summon a mountain of wealth to sustain the covenant. Use a creo herbam ritual to create enough grain to feed an entire city. Summon a swarm of locusts to blight your enemies. Use a creo herbam ritual to bless a forest and ensure all plants grow healthy within it for a year. Summon a powerful spirit of nature, for whatever reason. etc. etc.
Depending on the circumstances of the saga and the particular magus none of the above might be worth the effort, but they aren't useless in a general sense.
I would add the the CrCo and CrMe spells to boost your stats and those of your your Covenant - mates.
The ritual learned as part of the Guardian of Nature virtue only takes an hour or so to perform, so vis from Wilding can be used for it, which makes assuming/resuming guardian status easier.
The maga or magus would need to have a plan for the vis. If you've got the virtue you follow it up with learning a number of ritual spells. Aside from Creo rituals to make stuff, high level Rego rituals could be used to change the world in significant ways, An extra 20 or 24 penetration could be used to cast a spell on formidable enemy without first getting an arcane connection (or combine both and start using duration year rego mentem spells on Archangels).
There is a way to preserve the wilding powered rituals, but it takes more vis. Watching Ward just specifies that it targets a spell. Unless the Story guide finds this abusive, you can put all of your Wilding vis rituals in magical containers for use in emergencies, providing that you can somehow get an amount of vim or rego vis as well.
I've never looked too closely at the nature mysteries so it's never occurred to me before, but a combination of hermetic theurgy and wilding could be very powerful. Use wilding to gather a lot of vis each season and use it on repeated invocations to summon a daimon of your choice (presumably nature themed, if you're already deep into nature mysteries).
Wilding makes sense as long as you have chosen a nature lore that matches your chosen magical speciality and choice of ritual spells well.
For example. I designed a Hibernian merinita who was focused on that Irish status symbol, cattle. Wilding allowed him to gather Animal vis to power rituals that targeted whole bloodlines of cattle (using the merinita bloodline target)
Other ideas of the top of my head would be a mountain lore specialist gathering terram vis for CrTe rituals to magic up buildings or metal objects; a sea lore specialist gathering the vis to cast a ritual targeting a huge area of sea, or a sky lore specialist doing the same with auram (the really big auram rituals in the core book are expensive).