When someone proposes a trade that involves Ray Allen, please do not show your ignorance by brow beating the Original Poster - we should assume they are fans, and are interested in seeing our team improve.
There are three types of teams in the NBA, and three types of assets.
Team Getting Younger
Team Shedding Salary
Team Competing for a Championship
Pick/Prospect
Expiring Contract/Cap Space
Good Player
The overwhelming majority of trades happen when teams in different phases have assets that do not help them - Pau Gasol (Good Player) on Memphis (Team Shedding Salary), or Al Jefferson (Pick/Prospect) on Boston (Team Competing for a Championship).
The Celtics are a Team Competing for a Championship, so we want Good Players. On the surface, that makes everyone in our starting lineup (All Good Players) untouchable. However, other teams might consider some of our players assets of a different sort - Ray Allen is a good player, but he is also an expiring contract. Perkins and Rondo are good players, but they are also Picks/Prospects
Teams Shedding Salary or Getting Younger may be willing to trade thier Good Players for our Expiring Contracts and/or Prospects at a rate that exceeds the level we would recieve in a Good Player for Good Player swap, because they value them differently.
Now, because our Good Players are old, we also value Picks/Prospects. That, and the fact that Hobbs might ban you for your own safety, is why few people on this board proposes trades that involve Rondo or Perkins. Pierce and Garnett are Good Players, but are not assets in other ways, and are therefore unlikely to be traded for other Good Players (Assuming the return would be neutral).
A lot of people on these boards, including myself, are Ray Allen fans. However, I am a Celtics fan first. If we can trade him as an Expring Contract and return Good Players at a value that exceed his, I am all for it. Because we are all C's fans here, and we all want to see this team improve and compete, lets show mutual respect for other posters proposing ideas to that might help this team(Regardless of how innance some of the proposals might be - like my Mike Dunleavy & Jeff Foster for Ray Allen proposal from last summer... ugh).