I have noticed that some agents (mostly L8) will attack my portals but then they leave them unclaimed. Is this some kind of strategy or do they run out of XM? This has happened many times so the latter is unlikely.
The reasons could differ from agent to agent. Sometimes it could be because the agent is on a rush, maybe dropping xmp while driving or while on the bus, other times because you don't have resonators of the appropriate level (L4 resonators for example) and you prefer not to waste L8 and L7 resonators on an incomplete portal that will be easily destroyed 5 minutes later.
Often high level agents do this to help low level recruits to gain AP, but everytime I've seen doing this they put at least a L8 resonator to make an eventual attacker waste at least a couple of XMP to regain the portal.
As far as I know there's no actual strategy behind this, just personal decisions of the single agents.
Kappei's answer may be correct, but in terms of roleplaying this is more suitable:
The Third Faction
There are rumours that there is a third faction, its colour is white. Anyone can be part of it, be it a green or a blue player, since it's not about the colour but a state of mind.
There isn't actually such faction, but players who tend to leave neutral portals behind often claim they are part of this non-existent faction.
These players are the most honourable players ever, their lives and gameplay resembles lives of Hermits.
Affiliation with this faction isn't for everyone and is highly prestigious. (It's not something to be ashamed of!)
When a player acquires certain knowledge, or one could say 'enlightenment' (do not confuse with the green faction), he may decide to join this faction and fight for their beliefs by purifying all portals.
A player joins this faction usually when he has reached his mission and no more desires to be an agent, such as accumulate 20M AP, submit 500 portals, walk 2000 km, visit 10k portals etc.
I have two main reasons for doing this:
I tend to be in a hurry. I get a bit of hacking and bursting done on my commute, and a little during lunch breaks and weekends. So if I'm out for lunch I am not going to hang around after smashing a portal. I don't need the AP. In fact most of the time I only take out key portals - ie ones heavily mitigated and shielded, as L6 and L7 agents can quite happily manage lower level portals.
The single most important thing an L8 in a particular area can do is level up new recruits, as they are what makes the difference. So I'll liaise with one or two low level folks and lead them through. Can get a newbie up to at least L4 in no time.
As a L9 now, I only 'tag' (1Res) new portals for the 'unique' stat unless I'm looking to use them to make new fields, as it takes 8xL8 to make them worth farming, and the opposition is 4x as numerous so they take them down ASAP they go up in my area anyway.
I use L8 resonators (or L7 if more stock) as these are only 1-per-portal, so pretty difficult to run out of anyway :)
Once you reach Level 7 or so, you find that it's way less effort to earn AP by taking down portals anchoring lots of fields (3000-5000AP in a minute or two) than it is to deploy new portals or field them (10-15 minutes). Add to this the huge range of L8 XMPs: when trying to knock out a single well-defended portal, often I take out as many as 6-8 others in the area as collateral damage, and I can't be bothered to walk around to and redeploy all of them.
Individual portals have no value; the Global and regional stats are based upon field-MU.
I am over level 8, and have no need for AP or further levels, so I leave them open for others to level up with.
I have no resonators as I drop everything except high-level XMP and VR(+) items.
When I do add resonators, the portal stays my color too long and I prefer to allow an enemy to deploy there so I can blast their resonators.