Absolutely wrong.
I can't count the number of kills I've had defending my team's S&R location.
I've lost count of my much easier it is to hold a C&H location when you can hide cloaked and easily pick off anyone who is capturing.
For assassination defense, marksmen provide a great barrier between the enemy and the target. Enemies will often sprint right up to you without realizing you are there. Mow them down.
For assassination offense, snipers are probably the biggest worry people have, which is why they hide behind buildings or in rooms.
For bodycount, we all know how easy it is to get a great KDR using the marksman class.
For search and destroy offense, I prefer to go infiltrator until we get a plant, and if we have the advantage, I'll go marksman because a marksman can guard the plant zone better than anyone. I get a lot of kills doing this.
For search and destroy defense, obviously snipers make great defenders, and a sniper sitting on the bomb locations can take out most planters.
Invisibility is extremely useful when going after objectives. There are two ways to skin a cat. Simply planting the bomb isn't all that is needed to win S&D, and capturing the speaker isn't all that is needed to win S&R, etc etc.