Of course it is.
And thanks Cerovan for the answer: it is really comfortable to realize we are listened.

If your only problem is about the players who don't have all the environments, i think it shouldn't be a no-go, there is for sure a solution that can satisfy everyone.
Tcq's idea of a filter that would hide the servers you don't have all the titles to play on is good, in my opinion.
Players could have the opportunity to remove this filter through a discret button, and then join servers that run titles they can't play. Then, as Smigles suggested, they would be kicked if they don't get the title of the current map. While disabling the filter, they could get also a warning message such as "
Caution: while disabling this filter, you'll access servers that run titles you didn't buy. If such a map is played, you will be auto-kicked of the server", so that peoples are warned about what is likely to happen. Maybe better explained.
An advertisement for the last title the player didn't bought could be joined to the warning message (could be habile

).
Maybe your concern is also to be sure they'll be enough players/servers playing each title separately, so that the gaming experience is satisfying even if they don't buy the other games. I think the distribution of servers will actually follow the distribution of players. For instance, if players that bought only Canyon have very few servers, because almost all servers are Valley+Canyon, they'll join the few crowded Canyon only servers. But if they are not stupid, they'll quickly understand their community needs more servers and they'll build them. Reversely, if Canyon only servers are empty, the only reason possible is that there are very few Canyon-only players. Then, their gaming experience might be a bit less interesting, but several things are to highlight:
1. The number of players we're talking about is then low.
2. It is a good manner to encourage them to buy other titles.
3. It have allways worked like that. When i bought TMS, it soon have been left for TMU, and i had to buy TMU to follow the whole community. Nadeo didn't feel forced to maintain TMO and TMS separated in order to avoid people that have got only TMO or TMS to feel alone...
Anyway, when a "single-environment only" community is large enough, they used to build their own servers with only that environment. Coast and Island servers on TMU proved that it is likely to happen even if multi-environments servers are open as well. And lots of players actually play both types.
All in all, i think the pros are more powerful than the cons... I hope i convinced you ^^
PS: Some people say multi-environments server would need to create a title that merges all the environments, and that low performances PCs couldn't handle such an heavy title. Do you confirm this hypothesis?