I think if they give you a reasonable window of time to execute the combo it should be fine. Some people might believe strongly that you shouldn't have that much time to set up the combo. but I think you should be given ample time in order for other members to catch on to what you're doing and then follow up with the proper thing without even saying anything.

People did this with skillchains in FFXI. Experienced players had the skillchains memorized and could follow up someone's weaponskill with the proper WS to do a skillchain without even planning it ahead of time. I think eventually people will get to this point in FFXIV as well if a combo system is developed. However I think much more time should be allowed to perform the combo in FFXIV. I'd suggest a time limit of a full minute. The reasoning for this is because many combos require the player to be in the proper position in order to execute. Without enough time for positioning people may have a hard time executing the combos. Also one minute would be enough time for multiple party members to realize what's happening, get tp, get in position, and execute the proper combo.