Think it's one of those things that can go either way depending on how it's done.
As in be careful how you go about it, you'll neeed a deft hand to pull it off.
I have been invited to hen dos a couple of times without being invited to the wedding.
The first time it happened, I went along thinking it'd be fine and a nice boozynight out. It was in a local Italian for a meal, followed by drinks and dancing, so I thought it would be a fun Saturday night. Then there was an option for a spa day at a local place the next day to soak out the hangover and I thought that would be fun too do I said I would go along to that.
But the bride proceeded to talk all night with her bridesmaids about arrangements/what everyone needed to wear and do on the day, so the divide between wedding guests and non-wedding guests was a bit obvious and it was all a bit tactless and not much fun.
The bride kept talking about the Louis Vuitton handbag she wanted her bridesmaids/friends/hens to club together to buy her as a "brides gift"(?) and it was clear that all the hens who weren't invited to the wedding were expected to contribute to that as they wouldn't be giving wedding presents.
She, and her bridesmaids, all proceeded to order lobster/fillet steak/champagne off the a large carte whilst everyone else ordered pizza and pasta with house wine off the specials menu. After the orders had been taken, the bride's sister came round to quietly talk to everyone about the fact that the bride shouldn't have to pay as it was her hen do and the bridesmaids shouldn't have to pay as they organised the hen-do and were doing so much to help organise the wedding. So then everyone who had ordered cheaply started frantically ordering cocktails or g&ts from the bar as they realised they were going to get stuck with a big bill anyway, be as well get sloshed.
Not surprisingly, it was a bit of s damp squib and after the meal everyone was skint and needed to get the last bus home made polite excuses about not being able to go on for drinks/dancing. So a few of us ended up back at our house (my flat mate was doing a reading at the wedding and felt sorry for the bride that her gen-night was going to end at 9!).
Loads of people also dropped out of the spa day but by text after we were back home. The brude's suster tried to keep maintaining that everyone should still split the bride's spa day between them- which had been fine with about fifteen people going, but once it was down to half a dozen (on top of paying for the fillet steak and Louis Vuitton handbag) people started getting twitchy about it. My flat mate just point blank said she could no longer afford it after how expensive the meal had been and would have to see if she could borrow money from her boyfriend to be able to go at all. I think she did that more to get the point across as the bride seemed a bit oblivious to the fact that people were dropping out like flies.
So next time I was invited to a hen do but not the wedding, I had been a bit put out y previous experience! Plus it was a weekend away in the bride's home city, so involved train fares, hotel rooms etc. I knew it would end up costing a good few hundred quid so I just politely declined as I couldn't afford it, as did a few other invitees. We all offered to go for a drink locally though, sometime between the wedding and the hen do (we were all work colleagues).
A few weeks later, I did get invited to the wedding, along with a kind of muffled explanation/apology. I knew it was partly down to people who were originally invited declining, but I did notice that all the hen's who declined got invited then, so it kind of worked itself out. Went along, took a nice gift, had a nice time pity the groom turned out to be a controlling arsehole and cheating git soon after.
So I think it really depends how you handle it- low key local hen night and small wedding plus a bit of tact, you'll be fine! Requests for Louis Vuitton handbags and spa days, not so much!