There are plenty of times during the slower time periods where it is a royal *pain* to get together a mission team in Pen/Inf (not counting issues of rare Pen teams due to huge Inf mish rewards).
More often than not, there are actually enough people looking for xp/sk to make a team, except for one problem: the combination of people would make a mixed clan/omni team, which is automatically no good for SL missions.
When SL missions were first introduced, this was not really a problem, since the SK/hr in outdoor sk was comparable to missions (pre-Inf reward), enabling mixed teams of clan/neut/omni to get SK together outdoors as an alternative to missions.
The problem lies in 2 areas: the faction-focused nature of SL missions, and the large difference in SK/hr attainable. The faction focus of SL missions prevents clan/omni from teaming in SL missions due to the large negative effect on redeemed/unredeemed faction that would result (neutrals obviously being unaffected), and the large difference in SK over time stops people from seeking outdoor SK as an alternative.
Even though outdoor SK is always available as it is now, the large difference in SK over time produces people who wait for ages to make a mission team for that big perceived payoff rather than go outside and at least get *some* SK.
As many predicted when the large Inf mission rewards were put into the game, outdoor leveling died nearly overnight in SL past Adonis (and in additional consequence Pen leveling largely died again as well).
Outdoor SK should either be boosted to be comparable SK/hr to missions again, or SL missions need to be adjusted to allow clan/omni to team together (ideally both). Even RK missions (even though they are sorely underused nowadays) allowed for leveling with mixed teams of opposing factions without any negative effects other than the occasional mob inside a mission that needed to be click-targeted/ctrl-tab'd.
While the forthcoming solo quests might present an alternative leveling method, it should also not be relied upon as the sole alternative source for decent SK.
P.S. On a side note, it would also be nice if Inf missions had their difficulty adjusted so that a hard mission is actually *hard* again, something that got nerfed around the same time the random room generation was added to SL missions to replace the static layouts (although I much preferred the static layout personally ;D).