The answer depends on a lot of factors.
Does your telesales / telemarketing team follow-up on every lead that is synched to SFDC?
Do these leads overwhelm the sales team and take them away from focusing on better qualified leads?
Are you capturing sufficient data about them and their company that would allow a lead scoring program or smart campaign to flag the ones that appear to be most qualified, so your sales team can focus just on those?
Generally we do synch all of the leads that engage with us to SFDC (e.g. from a tradeshow, webinar, download, etc.), but we provide qualification metrics so they can be prioritized. When we receive leads that have not engaged with us (e.g. from a sponsorship or a list purchase), we set the revenue stage to "Suspect" and assign them to me, so they don't show up in any of our sales team's lists.
We like to have all the leads including the "Suspects" in SFDC, so that if a salesperson decides to add a new Contact at an Account, they can search SFDC first to see if there is an existing Lead record and if so, they just convert the Lead and we avoid a duplicate being created. Plus they can see the communications that we've been sending to the lead and any actions taken by the lead.