Originally Posted by I'm Venom
A cameo of Andrew Garfield in-suit and out would be reinforcement for Sony's own movie - it's the surest way to draw attention. The general reaction to The Amazing Spider-Man wasn't positive.
But then casual audiences would think that TAS is part of the MCU and would wonder why he then can't appear in the Avengers with the others as part of the team. A lot of the general audience don't know who owns what, and don't necessarily realise that the Avengers is an actual team. They might just think that Marvel decided to throw all their existing film heroes together into one movie and not that there are specific people who form part of the team. It's just a team up movie to them.