The simple answer here is to vote with your wallet, and dont buy the damn DLC, and if its Project $10 content, just dont ever use it.
My experince with multiplayer effing up single player is Ratchet & Clank. R&C were great on PS2 and at one point they the multi player started crushing the single player game expirnce. Multi.P was introduced in the second which had a shorter story game, and by the 3rd, the single player was mearly short but sweet...., but mostly short.
Deadlock was pretty much all about multiplayer, and the single player was like a tack on more or less.
As for developing DLC durring game time, EA and Bioware have been doing that in an obvious way since ME2 and DA:O. Zaeed was clearly developed and cut soely for content for Project $10. Same thing with Shale in DA:O.
Developing DLC durring game dev time should be seen as a rip off. EA doesnt want to charge you $84.99 for Mass Effect 3 (well they do but); however, the rage for this price gougeing would cost them more sales. They have already listed the PC price at $60 bucks, and collecters at $70.
So instead of listing the game at a higher price, they excise portions of the game, levels, weapons, characters, etc. to sell after release to make up that extra money to get the price of the game up.
Selling decent but slightly overpriced DLC to customers makes the game cost more. Hell Iv bought probably another $25 dollers worth of ME2 DLC, most of it about 10 to 15% more than I thought it was worth. Thats on top of the collecters edition.
It is insulting to think that code and art is being created for the game but some asshat cuts it out just to sell it to you because they figure it will make them an extra dime.
Agreeing with game publishers and devs on this behavior will only reward them for creating 110% of the game in dev time and cutting off the extra 10% for sale post release. A more accurate description is that we infact get 90% of games from EA and the rest is doled out in 5% bits till we get the whole game, and then we start getting unplaned unpartialy developed DLC.