I know rain team is mostly offense ... so I figured putting up spikes or stealth rock would be a waste
I'm not sure I follow your logic here. What makes you think setting hazards on an offence team is a waste? The implication I'm getting from your post is that you'd be better off trying to break or pull a sweep, but entry hazards are vital part of achieving both those things. Yes, rain offence should include entry hazards, and it's absolutely worth setting them up.
Entry hazards are really important to offence teams as they bring down the percentage of damage needed to secure a KO, which means you rely less on damage rolls and can knock out Pokemon that might otherwise end your set-up. This sounds very surface-level, but it is very relevant to actual gameplay, and especially important for offence teams where the goal is to break through the opponent's team until one of your Pokemon has a clear path to sweep the rest.
This is one of an endless amount of examples I could use, but say that I'm running Kingdra and I'm trying to break my opponent's team for another Pokemon in the back to clean. My opponent has prepared Scarf Tapu Koko as their answer to rain, but only in response to the rest of their team being glaringly weak to the archetype. If I can remove Koko, I'm good to win the game because I've kept my win condition healthy in the back.
This is where I run into problems. Tapu Koko is faster than my Modest Kingdra and will 2HKO it if I don't knock it out first, which means I lose the opportunity to break an important threat. Can I do it? Well unfortunately, this outcome is not in my favour:
252+ SpA Life Orb Kingdra Hydro Pump vs. 0 HP / 4 SpD Tapu Koko: 261-308 (92.8 - 109.6%) -- 56.3% chance to OHKO
With the added chance I will miss in addition to the 46% of not getting a good damage roll, it's probable that Tapu Koko will beat Kingdra. How can I make sure that this important match-up for my team instead goes in my favour? Using entry hazards. If we deduct 12% from Tapu Koko from Stealth Rock, suddenly I'm in a great position to win the game:
252+ SpA Life Orb Kingdra Hydro Pump vs. 0 HP / 4 SpD Tapu Koko: 261-308 (92.8 - 109.6%) -- guaranteed OHKO after Stealth Rock
Now that I'm able to reliably beat Tapu Koko in this matchup, I have one further avenue through which my team will be able to win against enemy teams. As mentionted, this is applicable in an endless quantity of scenarios: entry hazards simply allow you to win more vital 1v1 matchups that can cause you to win games. Rain is no less reliant on entry hazards: wearing down constant switch-ins like Tangrowth and Ferrothorn with entry hazards lets you break through sooner and win more easily.
When Pokemon that fit so well into rain like Ferrothorn are available, there's little excuse for why you wouldn't use entry hazards. A Pokemon like Ferrothorn gets plenty of opportunities to set hazards because it forces so many switches -- just click rocks while the opponent switches to a counter, then try a double switch and begin a set-up. Then when you're next faced with a vital KO like the one with Kingdra, you'll get it because you got off the free prior damage.
To address your question with Pelipper, the decision with Defog depends on what set you're running. If you're using the Choice Specs pressure set, stay clear. But if you're running the defensive set with Damp Rock or Leftovers, Defog might be enticing over a non-essential coverage move like Hurricane. It'll depend more broadly on your team, though being rain offence the ability to remove Skicky Web and Toxic Spikes in particular is valuable.
