You may have to throw pessimism right up front. Have it on a macro.

"My concern is that if we fight harder enemies, they will link, we will die. We will lose time, not get XP, party gets discouraged and we lose members. Then we'll be in Ul'dah reforming. Should we take that risk?"

You can modify it any manner of ways to convey thy point.

"Yo! If we fight deese enemies, we be linkin' lot'sa hard enemies! Tank ain't high, DPS ain't high and we all outta moko grass sun. So plz, be a good lala. Stay hea, where da XP is safe... and sexy."

In seriousness, it seems to happen and the difference in enemy abilities is just to vast. There are links to consider and AoE damage avoidance. And the moment folks start playing hero (they go off to fight an enemy expecting healer support), it gets even worse. Especially higher level raptors.

*Shows several guys playing hero, surrounded by raptors. All the raptors charge up, none of them being focused on. Raptors unleash their weapon skills simultaneously. Tank/DPS have no chance to survive, healer make your time.*